Ezatiostat (TER199 free base; TLK199) is a tripeptide analog of glutathione and is a selective and orally active glutathione S-transferase P1-1 (GSTP1) inhibitor. Ezatiostat leads to JNK activation by inhibiting GSTP1. Ezatiostat stimulates both lymphocyte production and bone marrow progenitor proliferation. Ezatiostat has the potential for myelodysplastic syndrome (MDS) treatment.