CD44 and esophageal squamous cell carcinoma: The discovery that CD44+ cells isolated from ESCC cell lines display properties of cancer stem cells greatly stimulated the study of CD44 expression at the level of both normal and tumor esophageal tissues showing that: (a) in normal esophageal tissue CD44 expression was limited at the level of the basal compartment containing putative stem cells; (b) 70% of ESCC specimens exhibited a pronounced expression of CD44, with various patterns of positivity [105].