Type I protein arginine methyltransferases (PRMTs) catalyze uneven dimethylation of arginines on proteins. Type I PRMTs in addition to their substrates are actually implicated in human cancers, suggesting inhibition of type I PRMTs provide a therapeutic way of oncology. The current report describes GSK3368715 (EPZ019997), a effective, reversible type I PRMT inhibitor with anti-tumor effects in human cancer models. Inhibition of PRMT5, the predominant type II PRMT, produces synergistic cancer cell growth inhibition when along with GSK3368715. Interestingly, deletion in the methylthioadenosine phosphorylase gene (MTAP) results in accumulation in the metabolite 2-methylthioadenosine, an endogenous inhibitor of PRMT5, and correlates with sensitivity to GSK3368715 in cell lines. These data provide rationale to educate yourself regarding MTAP status just like a biomarker way of patient selection.