Screening combinatorial peptide libraries in vivo has uncovered a vascular address system that allows targeting of blood vessels. One hopes that direct analysis of homing ligands in patients and their corresponding receptors in tumor blood vessels (so called "ZIP codes") may form the basis of a targeted pharmacology. A central point is how can one validate these candidate ligand-receptor systems in the context of human cancer? Another often asked question is what happens to ZIP codes when tumors do metastasize? In this particular case, one such ZIP code, the interleukin-11 receptor, is expressed in a tumor stage-specific distribution in a clinically relevant panel of prostate cancer specimens, and mediates internalization of pro-apoptotic targeted ligands. The cover features homogeneous expression of interleukin-11 receptor in human prostate cancer metastatic to bone marrow. For details, see the article by Zurita et al. on p. 435 of this issue.