METTL9 is a protein-histidine N-methyltransferase that catalyzes 1-methylhistidine (π-methylhistidine) modification on target proteins containing a His-X-His (HxH) motif, where X is preferably a small amino acid 1. The enzyme recognizes the first histidine as a signature and specifically methylates the second histidine residue 2. METTL9 constitutes the main methyltransferase for 1-methylhistidine modification in cells 1 and methylates diverse substrates including S100A9, NDUFB3, SLC39A7, SLC39A6, ARMC6, and DNAJB12 1342. Methylation of these targets affects metal binding, particularly zinc acquisition 15. Mechanistically, METTL9 operates through both catalytic and non-catalytic functions. Its catalytic activity enhances mitochondrial respiratory Complex I activity and suppresses ferroptosis in mesenchymal stem cells via SLC39A7 methylation and PERK/ATF4 signaling 15. Non-catalytically, METTL9 sustains vertebrate neural development through secretory pathway modulation and Golgi integrity maintenance 6. Clinically, METTL9 downregulation associates with osteoporosis and sarcopenia 7, while elevated expression correlates with gastric cancer peritoneal dissemination 8 and colorectal cancer progression 9. Additionally, intestinal METTL9 secretion provides cross-kingdom antifungal defense against Candida species by methylating fungal zinc-scavenging proteins 10, with reduced mucosal METTL9 linked to inflammatory bowel disease 10.