CD19 and acute lymphoblastic leukemia: Focusing in on the cell of origin in KMT2A-rearranged infant B-ALL, key pieces of evidence are (1) rearrangement is prenatal event, as demonstrated by Guthrie card examinations and concordance in monozygotic twins14; (2) rearrangement in the hematopoietic compartment is observed in CD34+CD19− cells30, before VDJ recombination in most cases, resulting in low frequency of clonal immunoglobulin rearrangements31,32; and (3) rearrangement may also be seen in bone marrow mesenchymal cells, suggesting a prehematopoietic origin in some33.