PROS1 and cancer: In “innate chemoresistance”, the cancer cells possess some mechanisms responsible for the elimination of the cytotoxic effect of chemotherapeutic drugs (for review, see [81]) while in “acquired chemoresistance”, chronic exposure to the drug treatment is able to enrich the cancer cell population with a subpopulation possessing CSC specifications, such as the increased expression and/or activity of the cellular detoxification proteins.