The AP20 sub-region on 3p21.33 is most frequently rearranged (deleted, methylated) in several epithelial tumors, including lung, breast, and kidney cancers. To facilitate the identification of tumor suppressor genes, the authors constructed a 3.5 Mb physical and gene map of this region. They used NotI linking and jumping clones, STS PCR marker analysis, PFGE, and multicolor and fiber FISH to confirm the sequence order and map orientation. The authors unequivocally established the order of 28 STSs and 35 genes. Gaps between published BAC contigs were determined and covered by their own sequence data. The AP20 region was duplicated during mammalian evolution and homologous gene clusters were found in human chromosome 2 and syntenic mouse regions on chromosomes 1, 2, and 9. For details, see the article by Protopopov et al. on page 404 of this issue.