Because mice lacking p21 are also sensitized to the radiation-induced GI syndrome and to radiation-induced cardiac injury (Komarova et al., 2004; Kirsch et al., 2010b; Leibowitz et al., 2011; Lee et al., 2012), it is conceivable that lung cancers lacking p53 are less likely to respond to fractionated RT as compared to a large single-dose of RT. This evidence concerns the gene TP53 and lung cancer.