1A4I, 1DIA, 1DIB, 1DIG4522108156n/aENSMUSG00000021048P11586Q922D8NM_005956NM_001364837NM_138745NP_005947NP_001351766NP_620084Methylenetetrahydrofolate dehydrogenase, cyclohydrolase and formyltetrahydrofolate synthetase 1 (MTHFD1) is a gene located in humans on chromosome 14[4] that encodes a protein, C-1-tetrahydrofolate synthase, cytoplasmic also known as C1-THF synthase, with three distinct enzymatic activities.

[5][6][7] This gene encodes a protein that possesses three distinct enzymatic activities, methylenetetrahydrofolate dehydrogenase (1.5.1.5), methenyltetrahydrofolate cyclohydrolase (3.5.4.9) and formate–tetrahydrofolate ligase (6.3.4.3).

Each of these activities catalyzes one of three sequential reactions in the interconversion of 1-carbon derivatives of tetrahydrofolate, which are substrates for methionine, thymidylate, and de novo purine syntheses.

The trifunctional enzymatic activities are conferred by two major domains, an aminoterminal portion containing the dehydrogenase and cyclohydrolase activities and a larger synthetase domain.

[7][8] Mutations of the MTHFD1 gene may cause methylenetetrahydrofolate dehydrogenase 1 deficiency, also known as combined immunodeficiency and megaloblastic anemia with or without hyperhomocysteinemia (CIMAH).