Remarkably, copanlisib (0.01–1 μM) in combination with 0.05 μM lapatinib inhibited the growth of CSK- and PTEN-null OE19 cells as well as control cells in a dose-dependent manner (Fig. 4c), and similar results were obtained with another PI3K inhibitor, LY294002 (Fig. S4B), indicating that the PI3K pathway plays an important role in lapatinib resistance of CSK- and PTEN-null GC cells. This evidence concerns the gene PIK3CB and gastric cancer.