GSK3368715, a first-in-class, orally active, potent, and selective SAM-noncompetitive inhibitor of Type I Protein Arginine Methyltransferases (PRMTs), exhibits anti-tumor efficacy across multiple cancer models and alters exon usage with IC50 values in the lower nM range. It synergizes with GSK3326595 (Type II inhibitor) (Axon 3750) to inhibit tumor growth.