PROM1 and cancer: In the tumour microenvironment, activated Th1 lymphocytes and other CD133+CD44+ cancer stem cells (CSCs) stimulate in vivo the effector cytotoxic CTLs by secreting the key cytokine IL-2 and interacting through costimulatory molecules, i.e., MHC class II molecule, CD27, a member of the TNF receptor superfamily, and CD134, called TNF receptor superfamily member 4 (TNFRSF4) or OX-40 receptor [61].