O-acetylated sialic acids of glycoproteins and gangliosides show cell type-specific and developmentally regulated expression in various systems and sometimes reappear as oncofetal antigens. O-acetylation of sialic acids may influence cell-cell interactions mediated by the sialic acid-binding lectins. Here we describe the molecular cloning and sequencing of a gene that produces O-acetyl disialoganglioside (O-acetyl GD3). Expression analysis showed that this gene product participates the O-acetylation of GD3 ganglioside sialic acid. The deduced amino acid sequence of the product was found to be similar to the milk fat globule membrane glycoprotein.