In order to investigate the functional interplay between PAX5, RUNX1 and IKZF1 we studied the impact of combined heterozygote deletion of Pax5 in mouse Pre-B cells and expression of ETV6-RUNX1 or a functionally impaired IKZF1-protein (IKZF1DN) lacking all four Zn-fingers in the DNA binding domain which is commonly observed in B-ALL [24, 25]. Here, RUNX1 is linked to precursor B-cell acute lymphoblastic leukemia.