The function of Ensembl gene identifier ENSG00000127995 (CASD1, CAS1 domain sialic acid O acetyltransferase 1) is as follows. Key enzyme in the biosynthesis of O-acetylated (O-Ac) sialoglycans such as gangliosides O-AcGD3 and O-AcGD2, which affect various processes such as cell-cell interactions, host-pathogen recognition (PubMed:20947662, PubMed:26169044, PubMed:34208013). Catalyzes the transfer of an acetyl group from a donor, the acetyl- coenzyme-A molecule (acetyl-CoA), to the C7/8/9 OH-position of a sialic acid residue (PubMed:20947662, PubMed:26169044). The primary site of O- acetyl group transfer on sialic acid seems to depend on cell type and can be C7, from which the O-acetyl group could subsequently migrate to the C8 and then to the C9 position, or at C9 with possibility of migrating to the C8 and then to the C7 position (PubMed:20947662, PubMed:26169044). Together with ST8SIA1 (GD3 synthase) it increases the levels of ganglioside Ac-O-7-GD3 (PubMed:20947662). Can transfer the acetyl group from acetyl-CoA to free sialate (N-acetylneuraminate, Neu5Ac) in vitro, but has preferred substrate specificity for CMP- activated sialate (CMP-Neu5Ac), resulting in the formation of 9-O- acetylated CMP-Neu5Ac (CMP-Neu5,9Ac2) (PubMed:26169044). CMP-Neu5,9Ac2 may be used by sialyltransferases as a sialate donor for glycoconjugate acceptors such as ganglioside GD3 (PubMed:26169044). O-acetylation at position C9 of ganglioside GD3 can counteract the pro-apoptotic effects of the ganglioside GD3 in tumor cells (PubMed:12486096). {ECO:0000269|PubMed:12486096, ECO:0000269|PubMed:20947662, ECO:0000269|PubMed:26169044, ECO:0000269|PubMed:34208013, ECO:0000305|PubMed:20947662}.