PAX5 and precursor B-cell acute lymphoblastic leukemia: To investigate the role of PD-1 signaling in Pax5-dependent B-ALL, and to ascertain if, also in human B-ALLs, PD-1+ cells are functionally required for B-leukemogenesis, we used the PD-1 conditional knockout (Pdcd1fl/fl) mouse (Figure 3).