
[Cancer Research 60, 4037-4043, August 1, 2000]
© 2000 American Association for Cancer Research
Cancer Gene Discovery Using Digital Differential Display1
Daniela Scheurle,
Maurice Phil DeYoung,
David M. Binninger,
Holly Page,
Mohammad Jahanzeb and
Ramaswamy Narayanan2
Center for Molecular Biology and Biotechnology and Department of Biology, Florida Atlantic University, Boca Raton, Florida 33431 [D. S., M. P. D., D. M. B., H. P., R. N.], and Eugene M. and Christine E. Lynn Clinical Research Center, Boca Raton Community Hospital Cancer Center, Boca Raton, Florida 33486 [M. J.]
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ABSTRACT
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The Cancer Gene Anatomy Project database of the National Cancer
Institute has thousands of expressed sequences, both known and novel,
in the form of expressed sequence tags (ESTs). These ESTs, derived from
diverse normal and tumor cDNA libraries, offer an attractive starting
point for cancer gene discovery. Using a data-mining tool called
Digital Differential Display (DDD) from the Cancer Gene Anatomy Project
database, ESTs from six different solid tumor types (breast, colon,
lung, ovary, pancreas, and prostate) were analyzed for differential
expression. An electronic expression profile and chromosomal map
position of these hits were generated from the Unigene database. The
hits were categorized into major classes of genes including ribosomal
proteins, enzymes, cell surface molecules, secretory proteins, adhesion
molecules, and immunoglobulins and were found to be differentially
expressed in these tumor-derived libraries. Genes known to be
up-regulated in prostate, breast, and pancreatic carcinomas were
discovered by DDD, demonstrating the utility of this technique. Two
hundred known genes and 500 novel sequences were discovered to be
differentially expressed in these select tumor-derived libraries. Test
genes were validated for expression specificity by reverse
transcription-PCR, providing a proof of concept for gene discovery by
DDD. A comprehensive database of hits can be accessed at
http://www.fau.edu/cmbb/publications/cancergenes.htm. This solid tumor
DDD database should facilitate target identification for cancer
diagnostics and therapeutics.
 |
Introduction
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With the expected completion of the human genome sequencing
efforts in the next few years, over 100,000 new genes are likely to be
discovered (1, 2, 3)
. From these vast numbers of new genes,
new diagnostic and therapeutic targets for diseases like cancer are
predicted to emerge (4)
. Only a subset of genes is
expressed in a given cell, and the level of expression governs
function. High-throughput gene expression technology is becoming a
possibility for analyzing expression of a large number of sequences in
diseased and normal tissues with the use of microarrays and gene chips
(5, 6, 7, 8)
. A parallel way to initiate a search for genes
relevant to cancer diagnostics and therapy is to data mine the sequence
database (9, 10, 11, 12, 13)
. A large number of expressed sequences
from diverse organ-, species-, and disease-derived cDNA libraries are
being deposited in the form of
ESTs3
in different databases.
The CGAP database of the NCI is an attractive starting point for
cancer-specific gene discovery (13)
. The Human Tumor Gene
Index was initiated by the NCI in 1997 with a primary goal of
identifying genes expressed during development of human tumors in five
major cancer sites: (a) breast; (b) colon;
(c) lung; (d) ovary; and (e) prostate.
This database consists of expression information (mRNA) of thousands of
known and novel genes in diverse normal and tumor tissues. By
monitoring the electronic expression profile of many of these
sequences, it is possible to compile a list of genes that are
selectively expressed in the cancers. Data-mining tools are becoming
available to extract expression information about the ESTs derived from
various CGAP libraries (9
, 10
, 12 , 14)
. Currently, there
are 1.5 million ESTs in the CGAP database, of which 73,000 are novel
sequences. These sequences are also subclassified into those derived
from libraries of normal, precancerous, or cancer tissues. We chose the
DDD at the CGAP database to identify genes (both novel and known ESTs)
that are selectively up- or down-regulated in six major solid tumor
types (breast, colon, lung, ovary, pancreas, and prostate). Survey
sequencing of mRNA gene products can provide an indirect means of
generating gene expression fingerprints for cancer cells and their
normal counterparts. DDD is a computer method of comparing these
fingerprints. DDD is a quantitative method that enables the user to
determine the fold differences between the libraries being compared,
using a statistical method to quantitate the transcript levels. ESTs
present in tumor-derived libraries were compared against all other
libraries or against the corresponding normal libraries by DDD, and the
hits showing >10-fold differences were compiled for each of the organ
types. These hits were functionally classified into major classes of
proteins. Genes belonging to ribosomal proteins, enzymes, receptors,
binding proteins, secretory proteins, and cell adhesion molecules were
identified to be differentially expressed in these tumor types. A
comprehensive database of hits was created, providing additional
electronic expression data as well as novel ESTs that were thus
identified. This database can be accessed on the World Wide
Web.4
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Materials and Methods
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Data-mining of CGAP Database.
The CGAP database was
accessed,5
and the DDD tool was used according to the database instructions. DDD
takes advantage of the UniGene database by comparing the number of
times ESTs from different libraries were assigned to a particular
UniGene cluster. Six different solid tumor-derived EST libraries
(breast, colon, lung, ovary, pancreas, and prostate) with corresponding
normal tissue-derived libraries were chosen for DDD
(N = 110). To identify tumor- and
organ-specific ESTs, all the other organ- and tumor-derived EST
libraries (N = 327) were chosen for
comparison with each of the six tumor types. The nature of the
libraries (normal, pretumor, or tumor) was authenticated by comparison
of the CGAP data with UniGene
database.6
Those few libraries showing discrepancies of definition between the two
databases were excluded. The DDD was performed for each organ type
individually. DDD was performed using ESTs from tumors (pool A) and
corresponding normal organ (pool B) for DDD2 method or tumors
(pool A) and all other organ- and tumor-derived cDNA libraries
including the corresponding normal (pool B) for the DDD1 method using
the online tool. The output provided a numerical value in each pool
denoting the fraction of sequences within the pool that mapped to the
UniGene cluster, providing a dot intensity. Fold differences were
calculated by using the ratio of pool A:pool B. Statistically
significant hits (Fishers exact test) showing >10-fold differences
were compiled, and a preliminary database was created. Hits were
classified into major families using information generated from two web
sites.7
Novel ESTs were compiled into a separate database. The UniGene database
was accessed to establish an electronic expression profile (E-Northern)
for each of the hits to facilitate tumor- and organ-selective gene
discovery. The cytogenetic map position of the hits was also inferred
from the UniGene page. A final database of ESTs that were up-regulated,
down-regulated, and show absolute differences (+/-) in the six tumor
types was created.4
Validation of DDD Hits.
Tumor and normal tissues were obtained from the Cooperative Human
Tissue Network (Birmingham, AL). One µg of total RNA was
reverse-transcribed using random hexamers and Superscript Reverse
Transcriptase (Life Technologies, Inc., Gaithersburg, MD). One-fortieth
of the cDNA was PCR-amplified using gene-specific primers. Primers were
designed using the Primer 3 program on the
web.8
Primers were chosen for the following 13 known genes that showed
DDD specificity for colon tumors: (a) creatine kinase (Hs.
118843); (b) guanylate cyclase (Hs. 1085); (c)
ETS-variant gene (Hs. 179214); (d) placental lactogen (Hs.
75984); (e) troponin (Hs. 73980); (f) tinin (Hs.
172004); (g) fibrinogen (Hs. 90765); (h) homeobox
transcription factor 1 (Hs. 1545); (i) homeobox
transcription factor 2 (Hs. 7399); (j) myoglobin C
(Hs. 118836); (k) cytokeratin 20 (Hs. 84905);
(l) neurotensin receptor (Hs. 110642), and (m)
transmembrane glycoprotein (Hs. 143133). In addition, primers were
designed for 20 colon-specific novel ESTs. The primer sequences are
available on request. The PCR parameters included 94°C for 7 min,
followed by a 3540-cycle amplification at 94°C for 45 s,
62°C65°C for 45 s, and 72°C for 90 s, with a final
extension at 72°C for 10 min. RT-minus controls and genomic DNA
controls were routinely used to authenticate the RT-derived products.
One-half of the amplified products were separated by electrophoresis on
2% agarose gel and detected by ethidium bromide staining. Internal
control actin RT-PCR was done on all samples simultaneously.
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Results
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Electronic Profiling of Up-Regulated Genes in Solid Tumors.
The EST libraries representing each of the organ types used in
the DDD protocol were chosen from the CGAP database library browser.
The content of each of the libraries used was verified by comparison
against the UniGene database.6
The DDD was performed by
taking all the libraries representing tumors for each organ type and
digitally comparing against either the corresponding normal
tissue-derived library (DDD2) or all the other libraries plus the
corresponding normal in the database (DDD1). Pretumor libraries were
not included in all the other libraries and were analyzed separately
(DDD3 and DDD4). Comparing the tumor libraries with all of the other
libraries including the corresponding normal (DDD1) or with the
corresponding normal only (DDD2) resulted in the identification of over
600 ESTs (data not shown). These hits were subdivided into known and
novel ESTs. Approximately 10% of the hits showed varying levels of
similarity (weak, moderate, and high) to alu-containing
repeat sequences. An interesting pattern emerged regarding the known
ESTs thus identified. The majority of the known ESTs can be classified
into distinct classes of genes. These included ribosomal proteins,
enzymes, cell surface receptors, binding proteins, secretory proteins,
cell adhesion molecules, and immunoglobulins. Over 80 genes were found
to be up-regulated with >10-fold differences in comparison to normal
and all other organs (Table 1)
. Ribosomal proteins were found to be up-regulated in breast- and
prostate carcinoma-derived libraries, but not in the other four solid
tumor-derived libraries. Known hits (enzymes) for select organ type
(for example, prostate-specific antigen and prostatic acid phosphatase
for prostate, several pancreatic enzymes for pancreas, tryptophan
hydroxylase and DOPA decarboxylase in the lung, and mammaglobin
for breast) were identified by the DDD protocol. The DDD also
identified mucin-related proteins in the colon and folate receptor in
the ovary carcinoma-derived EST libraries. The majority of the
up-regulated genes were identified by both DDD1 and DDD2 protocols.
These results suggest the potential utility of the DDD protocol to
rapidly identify tumor-selective genes. An electronic Northern (sources
of cDNAs) and cytogenetic map position for each of the hits was created
from the UniGene page (data not shown). The above-mentioned DDD
approach also resulted in the identification of up-regulated novel ESTs
(data not shown). The vast majority of the ESTs were present in an
organ- and tumor type-dependent manner. The results of these hits and
additional hits (>5-fold), including the novel ESTs, can be viewed at
the web site.4
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Table 1 DDD of up-regulated genes in tumors
Hits (known ESTs) showing >10-fold differences in the indicated
tumor-derived libraries in comparison with normal tissue-derived cDNA
library and all other organ- and tumor-derived cDNA libraries were
compiled. ESTs belonging to specific classes of genes were
subclassified as indicated. UniGene number for each hit is shown.
Electronic expression (E-Northern) for each of these hits was inferred
from the UniGene database from the cDNA sources, and the chromosomal
map position for each of these hits was inferred from the cytogenetic
map.4
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Electronic Profiling of Down-Regulated Genes in Solid Tumors.
Using the same DDD protocol (DDD1 and DDD2), a list of genes that are
down-regulated by >10-fold was also compiled (Table 2)
. The majority of the down-regulated genes were discovered by DDD2
protocol (tumor versus normal). Similar to the
above-mentioned results, distinct members of ribosomal proteins,
enzymes, cell surface receptors, binding proteins, secretory proteins,
cell adhesion molecules, and immunoglobulins were discovered to be
selectively down-regulated in an organ- and tumor type-dependent manner
(N = 34). A complete listing of all the genes
that are down-regulated (>5-fold) with their E-Northern and
cytogenetic map position can be accessed at the web site.4
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Table 2 DDD of down-regulated genes in tumors
Hits showing >10-fold differences in normal organ-derived cDNA library
in comparison with tumor-derived and other organ-derived libraries were
compiled. The hits are classified as shown in Table 2
.4
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Electronic Prediction of Tumor-selective Genes by DDD.
Using the DDD1 and DDD2 protocol, a list of known genes that are
predicted to be either present or absent in the tumor types (plus/minus
differences) was compiled (Table 3)
. Fourteen genes were found to be present exclusively in the select solid
tumor-derived libraries (Table 3)
. These included mammaglobulin in
breast, androgen receptor in prostate,
-glutamyl transferase in
lung, and neurotensin receptor in pancreas. Seventeen genes were found
to be selectively absent in the solid tumors analyzed (Table 4)
. These included seminogelin in the prostate, apolipoprotein in the
breast, and islet ameloid polypeptide in the pancreas.
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Table 3 DDD to identify hits present in the tumor types
From the hits of Tables 2
3
, ESTs present only in the tumor were
compiled.4
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Table 4 DDD to identify hits absent in the tumor types
From the hits of Tables 2
3
, ESTs absent only in the tumor were
compiled.4
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Test Validation of DDD Hits by RT-PCR.
The electronic prediction by DDD was tested for expression relevance
using cDNAs from a matched set of normal and tumor colon tissues (Fig. 1
). Twelve known genes that showed plus/minus differences from the analysis
of colon DDD from Table 3
and Table 4
were analyzed by RT-PCR. Three of
these 12 genes showed concordance with the DDD prediction. The results
of these three genes are shown in Fig. 1
. Creatine kinase (DDD2)
expression was detected in the normal colon-derived but not tumor
colon-derived cDNAs, whereas, Guanylate cyclase (DDD7) and ETS variant
gene (DDD12) were expressed in the same tumor, but not in the normal
colon-derived cDNAs, whereas, RT-PCR analysis of the remaining nine
genes did not show the DDD-predicted specificity of expression (data
not shown). Furthermore, analysis of 20 novel ESTs predicted to be
specific for colon tumors by DDD demonstrated two of the ESTs to be
specifically expressed in the tumor, consistent with the prediction of
DDD (data not shown). The authenticity of the RT-derived products was
established using RT-minus reactions. These results demonstrated that
it is possible to rapidly validate the electronic prediction by DDD.

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Fig. 1. RT-PCR validation of test hits identified by DDD. Three
hits (creatine kinase MM-DDD2, guanylate cyclase 2C-DDD7, and ETS
variant gene 3-DDD12) from Table 3
were chosen for expression
specificity using a matched set of normal- and tumor colon-derived
random primed cDNAs synthesized in the presence (RT+) or absence of
(RT-) reverse transcriptase. NEG, template minus negative
control.
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Discussion
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Discovery of cancer genes is a major challenge facing cancer
research (9
, 13
, 15)
. It is becoming increasingly clear
that multiple genetic alterations are responsible for cancer
development. The task of identifying cancer genes is complex due to the
fact that only a subset of genes within a cell population is expressed.
Whereas in the past, cancer gene discovery followed conventional
methods such as mapping the gene, loss of heterozygosity, model
organism studies, and so forth, the future of cancer gene discovery has
to be able to make effective use of the large number of genes
(predicted to be around 100,000 per cell) that will emerge from the
human genome sequencing efforts (1, 2, 3, 4)
. These sequences
are being deposited in the vast sequence databases, most of which are
in the public domain. For cancer-specific gene discovery, the CGAP
database of the NCI provides a comprehensive collection not only of
expressed sequences in the form of ESTs but also of various data-mining
tools to analyze the ESTs. The basis of the CGAP database is
establishment of the molecular anatomy of the cancer cell by
determining the repertoire of genes that are expressed in the cancer in
a quantitative manner, so that a fingerprint can be established. By
comparing the fingerprints with the normal cells, it should be possible
to short list genes that are present in the cancer cells, which can be
followed up for relevance.
High-throughput gene expression techniques (microarrays, Genechips) to
identify cancer-specific genes are becoming available
(5, 6, 7, 8)
, however, the technology is not cost effective for
average laboratories. Furthermore, this method introduces a bias in
that only a limited number, usually one tumor- and normal-derived RNA,
can be used for the initial analysis. In view of the pharmacogenomic
gene expression profile differences that are usually seen in different
patients (8
, 9)
, direct use of this technique may become
limiting. In this context, data-mining the databases provides a
parallel approach to rapidly establish transcript-based fingerprinting.
The CGAP database currently offers three different data-mining tools:
(a) X-profiling; (b) SAGE analysis; and
(c) DDD. The X-profiling method enables identification of
mostly novel ESTs that are present or absent in a given library, but it
does not permit quantitation. The SAGE and DDD options enable
quantitation of transcript levels and identify both known and novel
ESTs. The SAGE database was a recent addition to the CGAP database
(10)
; currently, SAGE libraries are available for only a
few organs. Hence, to establish a proof of concept of cancer gene
discovery by data-mining, we choose the DDD protocol.
A database of ESTs, both known and novel, that are differentially
expressed at least >10-fold was created for six major solid tumor
types. An electronic Northern based on cDNA sources as well as the
cytogenetic map position was created for each of these hits. The vast
sequence database (1.5 million ESTs) was thus reduced to approximately
200 known genes and 500 novel ESTs for these six organ-derived tumor
types. When these hits were subdivided into major classes of genes, the
number of known hits was significantly reduced (1030 per organ type).
The majority of the hits were classifiable as ribosomal proteins,
enzymes, cell surface receptors, binding proteins, secretory proteins,
cell adhesion molecules, or immunoglobulins. Known organ type-specific
genes were identified, including prostate-specific antigens, prostatic
acid phosphatase, and androgen receptor for prostate; pancreatic
enzymes such as islet ameloid polypeptide, lipases, elastases, and
carboxypeptidases for pancreas; and mammaglobulin for breast. Our
recent demonstration of pancreatic tumor-specific expression of
neurotensin receptor (20)
was also corroborated by
the DDD results.
Distinct ribosomal proteins were found to be up- and down-regulated in
ovary-, pancreas-, and prostate cancer-derived libraries by DDD. A
recent report (17)
supports the selective involvement of
the ribosomal proteins in prostate and breast tumors. Subtractive
hybridization of prostatic hyperplasia from prostate tumors identified
distinct ribosomal proteins (L4, l5, L7a, L23a, l30, L37, S14, and
S18). Furthermore, L23a and S14 levels were shown to be elevated in
PC-3 prostate carcinoma cells in comparison to normal prostatic
epithelial cells (17)
. In addition, mutant p53 expression
has been shown to induce overexpression of selective ribosomal proteins
(17)
. These results demonstrate the utility of DDD for
rapid gene discovery of cancer.
Electronic expression profiling to identify cancer specific genes is
likely to have false positives. However, the true lead genes can be
rapidly validated by RT-PCR using appropriate cDNAs. RT-PCR validation
of test genes indicated the expression profile consistent with the DDD
prediction. We choose 12 known genes predicted to be either up- or
down-regulated in colon tumors by DDD. Three out of these 12 genes
showed the expected specificity of expression. Evidence in the
literature supports these three hits in select cancer types. Creatine
kinase isoforms have been shown to be lost in colon carcinomas
(18)
, consistent with our DDD findings. Similarly, the
guanylate cyclase C isoform has recently been shown to be a biomarker
for advanced colon carcinomas (19)
. The selective
up-regulation of the ETS variant gene seen in the colon tumor, if
validated in a large sample size, provides a novel diagnostic and
therapy target for colon carcinomas. The ETS gene belongs to the ETS
oncogene family (16)
, which has DNA binding ability. In
addition, the DDD protocol predicted the expression specificity of
neurotensin receptor in pancreatic tumors. We have recently identified
this gene by independently data-mining the Unigene database and showed
the expression specificity in the pancreatic tumors (20)
.
Identification by the DDD protocol of several known genes that have
already been shown to be of diagnostic and therapeutic value in
prostate, breast, and pancreatic tumors suggests that it is possible to
discover novel genes from the list of novel ESTs that we have
uncovered. In support of this, our preliminary results with 20
colon-specific novel ESTs have led to the identification of 2 ESTs
showing specificity of expression to the colon tumors. These novel ESTs
can be subjected to additional data-mining tools [e.g., comparison
with SAGE and X-profiling; contig construction to expand the sequences
and motifs recognition, and electronic Northern (14)
] to
further reduce the numbers before laboratory validation with relevant
cDNAs.
The ability to reduce the number of hits from the vast and
rapidly growing amount of sequence information in the sequence database
is crucial to efficient gene discovery. The results presented in this
report support the starting premise that by data-mining the CGAP
database, it is possible to rapidly short list both known and novel
ESTs for immediate follow-up studies. The database of hits we have
generated using DDD for six different solid tumor types should provide
a rapid starting point for discovery of both diagnostic and therapy
targets.
 |
Acknowledgments
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We thank the CGAP database for providing access and
the data-mining tools used in this study. We thank Jeanine Narayanan
for editorial assistance.
 |
FOOTNOTES
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The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
1 Supported by an institutional startup grant (to
R. N.). D. S. was supported by a grant from the Boca Raton
Community Hospital Foundation. Colon tumor and normal tissues were
provided by the Cooperative Human Tissue Network, which is funded by
the National Cancer Institute. 
2 To whom requests for reprints should be
addressed, at Center for Molecular Biology and Biotechnology, Florida
Atlantic University, 777 Glades Road, Boca Raton, FL 33431. Phone:
(561) 297-2018; Fax: (561) 297-2099; E-mail: Rnarayanan{at}fau.edu 
3 The abbreviations used are: EST, expressed
sequence tag; CGAP, Cancer Gene Anatomy Project; DDD, Digital
Differential Display; RT, reverse transcription; Hs., human sequence;
NCI, National Cancer Institute. 
4 http://www.fau.edu/cmbb/publications/cancergenes.htm. 
5 http://www.cgap.gov. 
6 http://www.ncbi.nlm.nih.gov/UniGene/. 
7 http://www.ncbi.nlm.nih.gov/Omin/ and the
GeneCards site http://bioinformatics.weizmann.ac.il/cards/. 
8 http://www.genome.wi.mit.edu//cgi-bin/primer/primer3_www.cgi. 
Received 2/ 2/00.
Accepted 6/ 8/00.
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C. Palena, D. E. Polev, K. Y. Tsang, R. I. Fernando, M. Litzinger, L. L. Krukovskaya, A. V. Baranova, A. P. Kozlov, and J. Schlom
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F. Qiu, L. Guo, T.-J. Wen, F. Liu, D. A. Ashlock, and P. S. Schnable
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S. Nelander, P. Mostad, and P. Lindahl
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M. P. DeYoung, M. Tress, and R. Narayanan
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M. Koslowski, O. Tureci, C. Bell, P. Krause, H.-A. Lehr, J. Brunner, G. Seitz, F. O. Nestle, C. Huber, and U. Sahin
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J. L. Dennis, J. K. Vass, E. C. Wit, W. N. Keith, and K. A. Oien
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H. S. Judelson and S. Roberts
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A. Ghadersohi and A. K. Sood
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