While CUX1 knockdown is synthetic lethal to many cancer cells and reduces survival of all cancer cells following ionizing radiation (Figure 1), CUX1 is not essential in normal cells as demonstrated from lethality screens in human cells [96], , the lack of effect of CUX1 knockdown in the DLD-1 derivative cell line, DKO-4, in which the KRAS oncogene has been removed [53], and the viability of Cux1−/− knockout mice [97–99]. This evidence concerns the gene KRAS and cancer.