Function: C1s B chain is a serine protease that combines with C1q and C1s to form C1, the first component of the classical pathway of the complement system. C1r activates C1s so that it can, in turn, activate C2 and C4. Disease: Defects in C1S are the cause of complement component C1s deficiency (C1SD) [MIM:613783]. A rare defect resulting in C1 deficiency and impaired activation of the complement classical pathway. C1 deficiency generally leads to severe immune complex disease with features of systemic lupus erythematosus and glomerulonephritis. Similarity: Belongs to the peptidase S1 family. Contains 2 CUB domains. Contains 1 EGF-like domain. Contains 1 peptidase S1 domain. Contains 2 Sushi (CCP/SCR) domains. PTM: The iron and 2-oxoglutarate dependent 3-hydroxylation of aspartate and asparagine is (R) stereospecific within EGF domains.