MUC6 and gastric cancer: Moreover, according to Nguyen et al., ATRA induces the expression of GCSCs differentiation markers such as cytokeratin 7 (KRT7), osteopontin (SSP1), cytokeratins (PanCK), mucin 6 (MUC6), and trefoil factor 3 (TTF3) in tumorspheres from two GC cell lines and in mice with subcutaneous tumor xenografts of two GC cell lines and two patient derived xenografts (PDX) [19].