We have also subsequently cloned the patient clone-specific ETV6::RUNX1 genomic fusion from the overt ALL cells in twin 1A and, by Q-PCR and sequencing, confirm that the same or identical sequence is present in CD19 + cells of the healthy co-twin 1B (Fig. 1C) and in the neonatal blood spots of both twins. This evidence concerns the gene CD19 and acute lymphoblastic leukemia.