PRMT9 (protein arginine methyltransferase 9) is an arginine methyltransferase that catalyzes both monomethylation and symmetric dimethylation of arginine residues in target proteins 1. Its primary function involves regulating pre-mRNA alternative splicing through symmetric dimethylation of splicing factor SF3B2 at arginine 508, which affects protein-RNA interactions at pre-mRNA anchoring sites critical for RNA splicing 1. PRMT9 also plays crucial roles in immune regulation by methylating MAVS at Arg41 and Arg43 to prevent spontaneous aggregation and maintain innate immune homeostasis 2, and by promoting degradation of STAT1 through symmetric dimethylation, thereby suppressing excessive M1-like macrophage inflammatory responses 3. Disease relevance is significant, as bi-allelic loss-of-function variants in PRMT9 cause a syndromic form of intellectual disability characterized by global developmental delay, autism spectrum disorder, epilepsy, and hypotonia 4. The G189R mutation abolishes methyltransferase activity and causes abnormal synapse development through dysregulated RNA splicing 1. Clinically, PRMT9 represents a potential therapeutic target in cancer, as its inhibition suppresses cancer stem cell maintenance and activates anticancer immunity 5. PRMT9 also shows promise in cardiovascular disease, where macrophage-specific overexpression improves cardiac function after myocardial infarction 3.