PROM1 and cancer: Interestingly, although the 5-FU treatment downregulated the expression of CD133, it is reported that CD133 expression is not restricted to cancer-initiating cells; further, during the metastatic transition, CD133+ tumor cells may give rise to the more aggressive CD133– subset (such as CD44+/CD24−/CD133−), taking away the necessity for CD133 in tumor initiation [27].