Catalyzes the removal of O-acetyl ester groups from position 9 of the parent sialic acid, N-acetylneuraminic acid. H2O + N-acetyl-9-O-acetylneuraminate = acetate + H(+) + N-acetylneuraminate H2O + N-acetyl-4-O-acetylneuraminate = acetate + H(+) + N-acetylneuraminate Widely expressed with high expression in the testis, prostate, and colon. Disease susceptibility is associated with variants affecting the gene represented in this entry.