In contrast, overexpression of PELI1 dramatically impaired the clonogenic ability in response to different doses of irradiation and thus promoted IR‐induced apoptosis of these tumor cells (Fig. 4A–E), enhanced IR‐induced cleavage of caspase 9, caspase 7, caspase 3, and PARP in TE‐1 and ECA‐109 tumor cells both in vitro and in vivo, and greatly decreased the tumor size after radiotherapy (Fig. 4F–I). This evidence concerns the gene CASP9 and neoplasm.