To expand that study and identify the immunophenotype of cell of origin of PFGs, in this study we sorted UCB MNC from 4 ALL pediatric patients with and without TEL/AML1 into 8 HSPC subpopulations using FACS, expanded them on a feeder of mesenchymal stem cells and consequently analyzed for the presence of TEL/AML1, BCR/ABL and MLL rearrangements by fluorescent in situ hybridization (FISH). The gene discussed is RUNX1; the disease is acute lymphoblastic leukemia.