PER2 and cancer: Interestingly, mutation of the PER2 gene in oncogenic cells was also shown to impart resistance to common chemotherapeutic drugs including methotrexate, gemcitabine, etoposide, vincristine and oxaliplatin by up-regulating the level of the gene encoding for aldehyde dehydrogenase 3a1, whose suppression of cytotoxic effects of anti-cancer drugs could be ascribed to the prevention of reactive oxygen species accumulation [51].