AXL and small cell lung carcinoma: Other factors such as small-cell lung cancer transformation, hepatocyte growth factor (HGF) overexpression, Gas6-Axl activation, Loss of phosphatase and pensin homolog (PTEN), epithelial–mesenchymal transition (EMT) and activating mutations in phosphatidylinositol-4,5-bisphosphate 3- kinase (PI3K) have also been reported to associate with acquired EGFR-TKI resistance [24–29].