PIK3CA and gastric cancer: To establish a cell-based system to probe the interaction between activated PI3K/AKT/mTORC1 signaling and loss of CBS expression, we first examined CBS protein expression in six gastric cancer cell lines compared with an SV40-transformed gastric epithelial cell line GES-1, which was derived from fetal stomach mucosa and was non-tumorigenic in nude mice (Ke et al., 1994).