(a–d) MEF WT or S6K-/- cell populations transduced with retroviruses encoding either empty vector pBABE (pB(-)), S6K1, or Iso-2 were irradiated with ionizing radiation 2.5 Gy (IR) (a). Cells were stained with DAPI, anti-phospho-histone H3 Alexa488, anti-Cyclin B1 Alexa647 and anti-Cyclin A PE561. G1, S, and G2 separation displayed according to DAPI staining (b). Dual analysis of pH3 and DAPI distinguishes M phase (c). Plot was gated on S/G2/M. Dual analysis of cyclin A and cyclin B1 distinguishes between S, G2, M (d). (e,f) Cell cycle FACS analysis of S6K-/- cells expressing retroviruses encoding S6K1 transfected with HA-tagged WT CDK1 or HA-tagged mutant CDK1 39 A. Thirty-six hr after transfection cells were treated with IR 5 Gy. After 24 hr, the cells were subjected to FACS analysis (e). The percentage of cells in G1, S and G2/M are depicted in the graph(f). * p<0.05, p values were calculated using Student’s t-test (two-tailed) from three biological replicate.