The cytosolic 5'-nucleotidase II gene (NT5C2), encodes a 5'-nucleotidase that is responsible for the inactivation of nucleoside-analog chemotherapy drugs. NT5C2 dephosphorylates and inactivates 6-thioinositol monophosphate and 6-thioguanosine monophosphate which mediate the cytotoxic effects of 6-mercaptopurine (6-MP) and 6-thioguanine (6-TG), two nucleoside analogs commonly used in the treatment of ALL. Activating mutations of NT5C2 have been reported in 19%of relapsed T cell ALLs and 3-10% of relapsed B-precursor ALLs. NT5C2 mutant proteins show increased nucleotidase activity and conferred resistance to chemotherapy with 6-mercaptopurine and 6-thioguanine, suggesting that relapse-specific mutations in NT5C2 may act as a mechanism of resistance to 6-MP and a genetic driver of relapse in ALL.