RUNX1 and acute lymphoblastic leukemia: To evaluate the impact of heterozygous RUNX1 mutations on TCRβ rearrangements in T-ALL, 13 DNAs from RUNX1 mutated T-ALLs (all heterozygous, 10 RHD domain, 3 TAD domain) were compared to 11 RUNX1 wildtype T-ALLs, matched for an immature (IM) stage of maturation arrest, as defined by the absence of cytoplasmic or surface TCRβ protein expression31.