Although FLT3LG mutations are rare in haematological malignancies and have not been previously reported in ALL, mutation of its receptor, FLT3, occurs in 10–15% of ALL.3 Both FLT3LG mutant samples harboured mutations in other RAS pathway genes (patient 3, KRAS, NRAS (n=2), PTPN11; patient 9, NRAS (n=4)), suggesting that, similar to other components of the RAS pathway, FLT3LG mutations coexist with other RAS gene mutations in iAMP21-ALL. Here, FLT3 is linked to acute lymphoblastic leukemia.