2-C-methyl-D-erythritol 4-phosphate + CTP + H⁺ = 4-CDP-2-C-methyl-D-erythritol + diphosphate (according to UniProt)

CTP + 2-C-methyl-D-erythritol 4-phosphate = diphosphate + 4-(cytidine 5'-diphospho)-2-C-methyl-D-erythritol (according to Swiss-Prot)