53BP1 accumulates within the PML-NBs and is recruited into IRIF after DSBS induction, promoting the activation of the repair signaling.33 Therefore, we studied the DSB kinetics by counting the number of 53BP1 foci in primary APL cells and NB4 and NB4-MR4 cells after 0.5, 3, and 24 h from irradiation with 1 Gy. This evidence concerns the gene PML and acute promyelocytic leukemia.