Mutations of PIK3CA were found in EGFR-TKI-resistant NSCLC cell lines, MET-TKI capmatinib-resistant NSCLC cell lines, regorafenib-resistant CRC cell lines, KIT/PDGFR-TKI-resistant GIST cell lines, imatinib-resistant Ph+ ALL cell lines, and HER-TKI neratinib-resistant HER2+ breast cancer cell lines,92–97 resulted in enhancing PI3K kinase activity. The gene discussed is PIK3CA; the disease is breast cancer.