CD44 and gastric cancer: Cell surface proteins are used as CSC markers for different types of tumors, for example, cluster of differentiation 34-positive (CD34+)/CD38 − for leukemia cells, CD13/CD45/CD90 for liver cancer, CD117/CD90/epithelial cell adhesion molecule for lung cancer [23, 24], and CD44 for colon and gastric cancers [25].