However, CD133 is also expressed in differentiated epithelial cells in various organs, as well as in hematopoietic cells.9 From its first use for identification of cancer stem cells in brain tumours10, CD133 is still the most commonly used brain cancer stem cell marker, despite the many contradictions regarding the methods used to detect the expression of a surface marker in brain tumours. The gene discussed is PROM1; the disease is cancer.