O-acetyltransferase that catalyzes 9-O-acetylation of sialic acids (PubMed:20947662, PubMed:26169044). Sialic acids are sugars at the reducing end of glycoproteins and glycolipids, and are involved in various processes such as cell-cell interactions, host-pathogen recognition (PubMed:20947662, PubMed:26169044). acetyl-CoA + N-acetylneuraminate = CoA + N-acetyl-9-O-acetylneuraminate acetyl-CoA + N-acetylneuraminate = CoA + N-acetyl-7-O-acetylneuraminate Highly expressed in peripheral B lymphocytes. N-glycosylated. Belongs to the PC-esterase family. CASD1 subfamily. Truncated N-terminus.