The function of METTL9 (methyltransferase 9, His-X-His N1(pi)-histidine, Ensembl gene identifier ENSG00000197006) is as follows. Protein-histidine N-methyltransferase that specifically catalyzes 1-methylhistidine (pros-methylhistidine) methylation of target proteins (PubMed:33563959, PubMed:34562450, PubMed:37015930, PubMed:37398635). Specifically methylates the second His of proteins with a His-x-His (HxH) motif (where 'x' is preferably a small amino acid), while exploiting the first one as a recognition signature (PubMed:37398635). Catalyzes methylation of target proteins such as S100A9, NDUFB3, SLC39A5, SLC39A7, ARMC6 and DNAJB12; 1-methylhistidine modification may affect the binding of zinc and other metals to its target proteins (PubMed:33563959, PubMed:34562450, PubMed:37015930, PubMed:37398635). Constitutes the main methyltransferase for the 1- methylhistidine modification in cell (PubMed:33563959). {ECO:0000269|PubMed:33563959, ECO:0000269|PubMed:34562450, ECO:0000269|PubMed:37015930, ECO:0000269|PubMed:37398635}.