Two related studies showed that cyclic hydroxamic-acid-containing peptide 31 (CHAP31) induced apoptosis in ESCC cells by inducing cleavage of caspase 9 and upregulation of the Bax/Bcl-2 ratio, sensitized ESCC cells to carbon ion RT and inhibited the expression of DNA repair-related genes [163, 164]. Here, CASP9 is linked to esophageal squamous cell carcinoma.