The activity of CDCP1 is regulated by limited proteolysis of its extracellular domain (ECD), converting the full-length (FL) 135 kDa glycoprotein to a 70 kDa membrane spanning carboxyl-terminal fragment (CTF) and an amino-terminal fragment (ATF) of 65 kDa that is shed from the surface of several prostate cancer cell lines 15,16 and detected in serum of colorectal cancer patients 17. This evidence concerns the gene CDCP1 and Familial prostate cancer.