PML and acute promyelocytic leukemia: To evaluate whether the residual γ-H2AX and 53BP1 foci/cell observed after 24 and 48 h from IR in APL blasts and PML-RARα-expressing cells may correlate with chromosome instability, the multicolor FISH (mFISH) analysis was performed in NB4 cells, either untreated or exposed to RA, in combination with exposure to 1 Gy.