Enhancement of cell death after combination treatment of ionizing radiation with phytosphingosine is mediated both by ROS generation and PARP-1 activation pathways. Clone no. 6 was treated with 10 Gy of radiation alone, 5 μg/mL of phytosphingosine alone, or combination of γ-radiation (10 Gy) and phytosphingosine (5 μg/mL) in the presence or absence of 10 mM NAC or siRNA for PARP-1. After 3 hours, cells were stained with Hoechst 33258, and apoptotic cells were quantitated by fluorescence microscopy. Results from 3 independent experiments are shown as means ± SEM.