CD34 and acute promyelocytic leukemia: By immunoblot analysis we further observed that γ-H2AX was undetectable or barely detectable in human CD34+ hematopoietic progenitor cells (HPCs) and CD34− mononuclear cells (MNCs), respectively, whereas high levels of γ-H2AX were observed in PML-RARα-expressing samples from three APL patients, as well as in NB4 and NB4-MR4 cells (Figure 2a).