To validate DNMT3A as a T-ALL tumor suppressor in human cells, guide RNAs (gRNAs) were designed to inactivate DNMT3A by CRISPR/CRISPR–associated protein 9 (CRISPR/Cas9) genome targeting in cells from patients with primary T-ALL with wild-type DNMT3A. Two previously validated gRNAs that target human DNMT3A29 were used alongside a negative control gRNA that targets the inert AAVS1 locus. The gene discussed is DNMT3A; the disease is acute lymphoblastic leukemia.