Cytidylyltransferase required for protein O-linked mannosylation (By similarity). Catalyzes the formation of CDP-ribitol nucleotide sugar from D-ribitol 5-phosphate. 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 (By similarity). 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 (By similarity). Not involved in dolichol production (By similarity). 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. Belongs to the IspD/TarI cytidylyltransferase family. IspD subfamily.