PROM1 and cancer: CD44, a transmembrane glycoprotein, is the first identified cell-surface marker used commonly for the isolation of CSCs [89], and subsequently, the associations of CD24, CD44v6, CD44v9, CD133, LGR-5, and cytosolic ALDH1 with the clinicopathological parameters of cancers have been investigated in various studies [19, 90–92].