Purity >96% by HPLC, NMR. 6-Thioguanine is a methyltransferase inhibitor. After incorporation into DNA, it disrupts cytosine methylation by DNA methyltransferases in vitro and acts as a DNA demethylating agent in vivo. It reactivates epigenetically silenced genes in acute lymphoblastic leukemia cells by facilitating proteasome-mediated degradation of DNA methyltransferase (DNMT1). Incorporates into DNA and induces double strand breaks which destabilize DNA structure resulting in cytotoxicity. Selectively kills BRCA2-defective tumors and overcomes PARP inhibitor resistance in a xenograft model. Anticancer and immunosuppressive activity.