Cytidylyltransferase required for protein O-linked mannosylation (PubMed:22522420, PubMed:27130732, PubMed:27601598, PubMed:26687144, PubMed:22522421, PubMed:26923585). Catalyzes the formation of CDP-ribitol nucleotide sugar from D-ribitol 5-phosphate (PubMed:27130732, PubMed:26687144, PubMed:26923585). CDP-ribitol is a substrate of FKTN during the biosynthesis of the phosphorylated O-mannosyl trisaccharide (N-acetylgalactosamine-beta-3-N-acetylglucosamine-beta-4-(phosphate-6-)mannose), a carbohydrate structure present in alpha-dystroglycan (DAG1), which is required for binding laminin G-like domain-containing extracellular proteins with high affinity (PubMed:27130732, PubMed:26687144, PubMed:26923585). Shows activity toward other pentose phosphate sugars and mediates formation of CDP-ribulose or CDP-ribose using CTP and ribulose-5-phosphate or ribose-5-phosphate, respectively (PubMed:26687144). Not Involved in dolichol production (PubMed:26687144). CTP + D-ribitol 5-phosphate + H(+) = CDP-L-ribitol + diphosphate CTP + D-ribose 5-phosphate + H(+) = CDP-D-ribose + diphosphate CTP + D-ribulose 5-phosphate + H(+) = CDP-D-ribulose + diphosphate Protein modification; protein glycosylation. Homodimer. Ubiquitously expressed, with high expression in brain. The disease is caused by variants affecting the gene represented in this entry. The disease is caused by variants affecting the gene represented in this entry. Belongs to the IspD/TarI cytidylyltransferase family. IspD subfamily.