Transports CMP-sialic acid from the cytosol into Golgi vesicles where glycosyltransferases function (PubMed:8755516, PubMed:10085119, PubMed:30985278, PubMed:34015330). Efficient CMP-sialic acid uptake depends on the presence of free CMP inside the vesicles, suggesting the proteins functions as an antiporter (PubMed:30985278). Binds both CMP-sialic acid and free CMP, but has higher affinity for free CMP (PubMed:30985278). Also mediates the transport of CDP-ribitol (PubMed:34015330). CMP(out) + CMP-N-acetyl-beta-neuraminate(in) = CMP(in) + CMP-N-acetyl-beta-neuraminate(out) CDP(out) + CDP-L-ribitol(in) = CDP(in) + CDP-L-ribitol(out) Monomer. Ubiquitous. Found in all the tissues examined including skeletal muscle, brain, heart, liver, kidney and spleen. Belongs to the nucleotide-sugar transporter family. SLC35A subfamily.