Protein arginine N-methyltransferase 1(PRMT1) is a type I methyltransferase that transfers a methyl group from S-adenosylmethionine to guanidino nitrogens of arginine residues to form monomethylarginine and asymmetric dimethylarginine. Functions of type I arginine methylation in proteins may include regulation of transcription, modulation of the affinity of nucleic acid-binding proteins, regulation of interferon signaling pathways, and targeting of nuclear proteins.