THY1 and esophageal squamous cell carcinoma: Nowadays, CD90 is considered as a marker for CSCs in gastric and esophageal squamous cell carcinomas and hepatocellular carcinoma (HCC) due to the ability of tumor-isolated CD90+ cells to generate cancer even upon the adoptive transfer of a very small number of these cells into immunodeficient mice compared to tumor-isolated CD90– cells (Yang et al., 2008; Jiang et al., 2012; Tang et al., 2013).