One pair of monozygotic twins (#2.4) who shared a unique vascular placenta developed ALL (both with the KMT2A-AFF1 fusion gene at the same time at 4 months of age), and the second pair (#3.2), who was dizygotic, had an independent placenta and a distinct leukemia type (24). This evidence concerns the gene AFF1 and acute lymphoblastic leukemia.