Single-nucleotide polymorphism (SNP) in TPMT (i.e., rs1142345) was first identified as a pharmacogenetic marker through pharmacology-guided approach, explaining the majority of 6MP-induced leukopenia cases in Caucasians and Africans (Relling et al., 1999; Moriyama et al., 2015). The gene discussed is TPMT; the disease is Decreased total leukocyte count.