IGHM and acute lymphoblastic leukemia: In addition, most ETV6-RUNX1 ALL expressed low mRNA levels of VPREB1 and/or IGLL1. One possible explanation is that the ETV6-RUNX1 fusion gene down-regulates VPREB1/IGLL1 expression [26], another is the combination of the fusion gene with mono-allelic deletions of VPREB1 and/or IGLL1. The reduced levels of VPREB1, IGLL1 and/or IGHM could also be as a result of genetic aberrations in transcription factors [1, 2], e.g. E2A, EBF and IKZF1 frequently found in BCP-ALLs, as these factors regulate their transcription and are crucial for B cell development [27–30].