Catalyzes the activation of N-acetylneuraminic acid (NeuNAc) to cytidine 5'-monophosphate N-acetylneuraminic acid (CMP-NeuNAc), a substrate required for the addition of sialic acid. Has some activity toward NeuNAc, N-glycolylneuraminic acid (Neu5Gc) or 2-keto-3-deoxy-D- glycero-D-galacto-nononic acid (KDN). This is the function of ENSG00000111726 (CMAS, cytidine monophosphate N-acetylneuraminic acid synthetase).