Regulation of O-acetylation of sialic acids by sialate-O-acetyltransferase and sialate-O-acetylesterase activities in childhood acute lymphoblastic leukemia. Enhanced expression of 9-O-acetylated sialoglycoproteins (Neu5,9Ac(2)-GPs) and 9-O-acetylated disialoganglioside (9-OAcGD3) wa