Isoform 1 and isoform 3 are predominantly expressed in lung, placenta, pancreas, and prostate. Isoform 3 is weakly expressed in testis and peripheral blood lymphocytes. The repeat A-S-P-A-[GLQR] is polymorphic and the number of copies varies between 12 to 14. Belongs to the peptidase S1 family. Was termed TMPRSS6 (Ref.3).