Our finding that the APOBEC signature of nucleotide substitution is only observed in patients with the ETV6-RUNX1 fusion gene and the previous demonstration of frequent RAG-mediated recombination in this subtype [18] suggests that different molecular mechanisms are active in patients with the ETV6-RUNX1 fusion gene than in the other ALL subtypes. The gene discussed is ETV6; the disease is acute lymphoblastic leukemia.