Evidence for such pre-malignant clones in ALL with BCR-ABL or other founder lesions have been provided by comparative genetics of monozygotic twins with discordant ALL.25, 26, 27 Sharing of identical or clonotypic BCR-ABL1 genomic fusions in monozygotic twins with concordant or discordant ALL but discordance of other genetic changes27 suggests that the BCR-ABL1 fusion in such cases is an early or likely founder or initiation event spawning a pre-leukemic clone. This evidence concerns the gene ABL1 and acute lymphoblastic leukemia.