Serine protease that has amidolytic activity, cleaving its substrates before Arg residues. Predominantly expressed in macrophages. Present in the spleen, small and large intestine, lung and brain (at protein level). Highly expressed in peripheral leukocytes, ovary, retina, spleen and stomach. Moderately expressed in thymus, uterus and platelets, as well as some brain tissues, such as thalamus and fetal brain. Up-regulated by phorbol myristate acetate (PMA). Belongs to the peptidase S1 family.