PAX5 was identified as a haploinsufficient tumor suppressor gene in human B‐ALL, as heterozygous PAX5 deletions, rearrangements, and loss‐of‐function mutations are present in one third of all cases (Kuiper et al, 2007; Mullighan et al, 2007). This evidence concerns the gene PAX5 and precursor B-cell acute lymphoblastic leukemia.