Nudix hydrolase 15 (NUDT15) deficiency is strongly associated with thiopurine-induced myelosuppression. Currently, testing for NUDT15 deficiency is based on the genotyping of the most frequent and clinically characterized no-function variants, that is, *2, *3 and *9. The Hispanic/Latino-predominant variant NUDT15 *4 (p.R139H) is classified as "uncertain function" by the Clinical Pharmacogenetics Implementation Consortium, because of insufficient data to ascertain its clinical actionability. In this study, we evaluated the association of NUDT15 *4 with mercaptopurine (6-MP) tolerance in a retrospective cohort of 1,399 patients with acute lymphoblastic leukemia (ALL) of diverse ancestries. All patients were wildtype for thiopurine methyltransferase gene. Patients were treated with 6-MP in the context of ALL frontline clinical trials. The tolerated dose of 6-MP was used to assess drug toxicity during the maintenance phase of ALL therapy. Patients with NUDT15 *1/*4 (n = 16, all of whom self-identified as Hispanic/Latino) tolerated a significantly lower dose of 6-MP than did those with NUDT15 *1/*1: median [interquartile range] of 39.0 [21.2-52.8] mg/m