2-C-Methyl-D-erythritol 2,4-cyclodiphosphate synthase (MEcPP synthase, IspF, EC 4.6.1.12) is a zinc-dependent enzyme and a member of the YgbB N terminal protein domain, which participates in the MEP pathway (non-mevalonate pathway) of isoprenoid precursor biosynthesis. It catalyzes the following reaction:^[1]

4-diphosphocytidyl-2-C-methyl-D)erythritol 2-phosphate ${\displaystyle \rightleftharpoons }$ 2-C-methyl-D-erythritol 2,4-cyclodiphosphate + CMP

The enzyme is considered a phosphorus-oxygen lyase. The systematic name of this enzyme class is 2-phospho-4-(cytidine 5′-diphospho)-2-C-methyl-D-D-erythritol CMP-lyase (cyclizing; 2-C-methyl-D-erythritol 2,4-cyclodiphosphate-forming). Other names in common use include IspF, YgbB and MEcPP synthase.

As of late 2007, 20 structures have been solved for this class of enzymes, with PDB accession codes 1GX1, 1H47, 1H48, 1IV1, 1IV2, 1IV3, 1IV4, 1T0A, 1U3L, 1U3P, 1U40, 1U43, 1VH8, 1VHA, 1W55, 1W57, 1YQN, 2AMT, 2GZL, and 2PMP.