Mediates the proteolytic cleavage of HP/haptoglobin in the endoplasmic reticulum. Highly expressed in placenta, liver, kidney, pancreas, moderately in lung, spleen, prostate, ovary, colon, and PBL, and weakly in heart, skeletal muscle, thymus, testis, and small intestine. Expressed in PC-3 (prostate adenocarcinoma) and SK-OV-3 (ovary adenocarcinoma) cells, but not in LoVo and HT-29 (colon adenocarcinoma), SMMC7721 (hepatocellular carcinoma), CaoV-3 (ovary adenocarcinoma), HeLa (cervix epithelioid carcinoma), MCF-7 (breast adenocarcinoma), U-251MG (glioma) or A-549 (lung carcinoma) cells. Widely expressed in myeloid leukemia cell lines, including K-562 (chronic myelogenous leukemia), THP-1 (myelomonocytic leukemia), HL-60 and NB4 (promyelocytic leukemia), and KG-1 (acute myelogenous leukemia) cells. Expressed mainly in the liver and in serum (at protein level). Up-regulated in monocytes and dendritic cells (DC) undergoing maturation or activation. Belongs to the peptidase S1 family. Does not associate with the C1 complex. According to PubMed:15385675, doesn't cleave the proform of complement C1s.