We have identified a gene located on chromosomes 21 that is expressed in normal and neoplastic prostate, and in normal testis, ovary, and placenta. We name this gene POTE (expressed in prostate, ovary, testis, and placenta). The POTE gene has 11 exons and 10 introns and spans approximately equal 32 kb of chromosome 21q11.2 region. The 1.83-kb mRNA of POTE encodes a protein of 66 kDa. Ten paralogs of the gene have been found dispersed among eight different chromosomes (2, 8, 13, 14, 15, 18, 21, and 22) with preservation of ORFs and splice junctions. The synonymous:nonsynonymous ratio indicates that the genes were duplicated rather recently but are diverging at a rate faster than the average for other paralogous genes. In prostate and in testis, at least five different paralogs are expressed. In situ hybridization shows that POTE is expressed in basal and terminal cells of normal prostate epithelium. It is also expressed in some prostate cancers and in the LnCAP prostate cancer cell line. The POTE protein contains seven ankyrin repeats between amino acids 140 and 380. Expression of POTE in prostate cancer and its undetectable expression in normal essential tissues make POTE a candidate for the immunotherapy of prostate cancer. The existence of a large number of closely related but rapidly diverging members, their location on multiple chromosomes and their limited expression pattern suggest an important role for the POTE gene family in reproductive processes.