RUNX1 and acute lymphoblastic leukemia: The RUNX1 mutated cases showed more diverse stages of maturation arrest, with 5 arrested prior to the onset of TCRβ rearrangement (3 IM0 with no clonal TCR rearrangement, 2 IMD with only TCRδ rearrangement), 3 IMG, 2 IMB, 2 pre-AB T-ALL (which expressed cytoplasmic but not surface TCRβ protein) and one which expressed a surface TCRγδ receptor (TCR-GD+).