Interestingly, we found that the mutation we identified, TCA to TTA, conforms to an APOBEC mutational signature, TCN to TTN, which has been widely identified across the genome in a variety of human cancer types as 'signature 2'.26, 27 APOBEC3 was highly expressed during thymocyte development in the mouse (Supplementary Figure S3A).28 There are 11 distinct human APOBEC family members.29 Human T-ALL cell lines expressed varying levels of APOBEC3B, 3C and 3G (Supplementary Figure S3B). The gene discussed is APOBEC3B; the disease is cancer.