The expression of CSC markers (SOX2, KLF4, NANOG, and CD133) was significantly higher in PC-9/GR and PC-9/pBTK-N1 cells (Fig. 4E, F), indicating that BTK may regulate the stemness of NSCLC cells to acquire the phenotype of Gefitinib resistance. This evidence concerns the gene KLF4 and non-small cell lung carcinoma.