Serine protease which preferentially hydrolyzes peptides with Arg at the P1 position. Forms a heterodimer with SERPINA5. Expressed in brain, eye, testis, skin, epididymis and salivary gland with lower levels in heart, skeletal muscle, thymus, ovary, prostate and uterus. N-glycosylated. Belongs to the peptidase S1 family. Differs at the N-terminus for unknown reasons.