Severe combined immunodeficiencies (SCIDs) are a group of hereditary diseases that involve severe dysfunction in the immune system (Cirillo et al. 2015). Impairments in metabolic pathways are recently considered as the underlying cause for specific types of immunodeficiencies. A subtype of SCID with megaloblastic anemia and neurologic abnormalities was reported in 2011 (Watkins et al. 2011). The immunodeficiency results from mutations in both alleles of MTHFD1 gene which encodes methylenetetrahydrofolate dehydrogenase 1, an important enzyme in folate metabolism cycle (Field et al. 2016; Pjetri and Zeisel 2017). This enzyme has three functions including 5,10-methylenetetra-hydrofolate dehydrogenase, 5,10-methenyltetrahydrofolate cyclohydrolase, and 10-formyltetrahydrofolate synthetase activity, and defects in each function lead to proliferation defect of lymphocytes (Burda et al. 2015). MTHFD1 deficiency is a rare type of SCID that only few cases have been reported so far, and patients usually are diagnosed in early life due to recurrent infections.