Thiopurine S-methyltransferase (TPMT) activity cutoffs in the Thai population - Pimonpan Jinda, Pimpun Kitpoka, Wimol Thienphopirak, Suwat Chiawchan, Santirhat Prommas, Rattanaporn Sukprasong, Jiratha Rachanakul, Supaporn Wiwattanakul, Orapa Suteerojntrak