Many studies on carcinogenesis carried out early in the last century are united on the consensus that cancer is a genetic disease. Cancer cells typically display gene dysfunction and endogenous or exogenous insults resulting in gene dysfunction are often carcinogenic. Recent advances in stem cell biology added the new concept that cancer originates from a single cancer-initiating cell. To understand the molecular basis of carcinogenesis from the beginning to the full acquirement of malignancy, factors concerned with carcinogenesis were categorized into three groups: those guarding and stabilizing genomes, those regulating cell proliferation, and those conferring resistance to various micro-environmental stresses. One example of particular interest is the Keap1-Nrf2 system since, according to recent studies, it has turned out to be ambivalent. Nrf2 heterodimerizes with small Maf protein to strongly activate transcription through the Maf recognition element (MARE) and Keap1 is an inhibitory regulator of Nrf2. The genes regulated by Nrf2 are very important for cellular protection of the genome from xenobiotic and oxidative stresses and, consequently, for preventing carcinogenesis. This implies that enhancing Nrf2 activity is a promising method for thwarting cancer. On the contrary, the constitutive activation of Nrf2 due to mutations in the keap1 gene is characteristically observed in lung cancer cells, suggesting that induced expression of Nrf2 target genes favors the prevalence of cancer cells.