Compositions for the diagnosis and therapy of prostate and colon cancer, derived from or based on a novel prostate-specific, androgen-regulated, cell membrane associated and secreted serine protease termed 20P1F12/TMPRSS2 are described. A full length cDNA comprising the entire coding sequence of the 20P1F12/TMPRSS2 gene (also designated 20P1F12-GTC1 herein) is provided (FIG. 1). Among the compositions provided are antibodies that bind to 20P1F12/TMPRSS2 proteins and polypeptide fragments thereof, including antibodies labeled with a detectable marker or toxin or therapeutic composition. The invention also provides prognostic and diagnostic methods of examining a biological sample for evidence of disregulated cellular growth by comparing the status of 20P1F12/TMPRSS2 in the biological sample to the status of 20P1F12/TMPRSS2 in a corresponding normal sample, wherein alterations in the status of 20P1F12/TMPRSS2 in the biological sample are associated with disregulated cellular growth. The invention further provides various therapeutic compositions and strategies for treating prostate cancer, including particularly, 20P1F12/TMPRSS2 polypeptide and anti-20P1F12/TMPRSS2 antibody therapy methods and compositions, cancer vaccines, and small molecule therapy.