Sialate O-Acetylesterase (SIAE) belongs to the family of hydrolases, specifically those acting on carboxylic ester bonds. SIAE is widely expressed with high expression levels in the testis, prostate, and colon. SIAE catalyzes N-acetyl-O-acetylneuraminate and H2O to N-acetylneuraminate and acetate. SIAE removes O-acetyl ester groups from position 9 of the parent sialic acid, N-acetylneuraminic acid. SIAE down-regulates B lymphocyte antigen receptor signaling (involving CD22), and is required for immunological tolerance. Loss of function mutations in SIAE are much more frequently found in humans with autoimmune diseases especially rheumatoid arthritis and type 1 diabetes.