Four cell lines in figure 3 show exceptional levels of regrowth delay after irradiation with single fractions of 15 Gy and these are: SW1222 (mutATM), 80S4 (wtp53, p21-), 19S186 (mutTP53, p21-) and U251 (radioresistant glioma "glio"). This evidence concerns the gene CDKN1A and central nervous system cancer.