O-acetyltransferase that catalyzes 9-O-acetylation of sialic acids. 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. acetyl-CoA + N-acetylneuraminate = CoA + N-acetyl-9-O-acetylneuraminate acetyl-CoA + N-acetylneuraminate = CoA + N-acetyl-7-O-acetylneuraminate Ubiquitously expressed. Expressed in neonatal brain and in day 10 and 13 embryo. N-glycosylated. The Casd1 locus is imprinted. Maternal inherited gene is expressed, while the paternal inherited gene is silenced. Belongs to the PC-esterase family. CASD1 subfamily.