MTHFD1L encodes a mitochondrial monofunctional 10-formyl-tetrahydrofolate synthetase that catalyzes the final step in converting one-carbon units from mitochondria to cytoplasm, producing formate from 10-formyl-tetrahydrofolate (THF) 1. This enzyme plays a critical role in linking mitochondrial and cytoplasmic folate metabolism, complementing MTHFD2 enzymatic activities. MTHFD1L is expressed throughout embryonic development with elevated expression in neural tube, brain, and craniofacial structures 1. Loss of MTHFD1L causes embryonic lethality with neural tube closure defects and craniofacial abnormalities 1, mechanistically linking the folate pathway to birth defects; maternal formate supplementation partially rescues this phenotype. Beyond developmental roles, MTHFD1L is dysregulated in multiple cancers. MTHFD1L is upregulated in papillary thyroid cancer and cutaneous melanoma, where knockdown inhibits proliferation and promotes apoptosis 23. Conversely, MTHFD1L is downregulated in colorectal cancer progression where it functions in folate cycle disruption 4. MTHFD1L polymorphisms correlate with increased cardiovascular disease risk 5, and MTHFD1L deficiency enhances ferroptosis induction by eprenetapopt in cancer cells 6. MTHFD1L retains epigenetic memory after high-intensity interval training with persistent hypomethylation 7.