PBX1 and precursor B-cell acute lymphoblastic leukemia: Second, the immunophenotypic and genotypic heterogeneity of ALL implies addiction to diverse survival signaling pathways, some of which may involve ROR1 as indicated by its uniform expression in the E2A-PBX1 subtype and our finding that >90% of primary B-ALL blasts from ROR1+ pediatric B-ALL patients express ROR1 with homogeneous cell surface density.