HGF and small cell lung carcinoma: Amplification of genes, such as MET and ERBB2, transformation to small cell lung cancer, hepatocyte growth factor (HGF) overexpression, EMT transition, and PIK3CA mutation/reduced PTEN expression are other events that can occur after treatment with first- and second-generation EGFR-TKIs and with cancer progression, leading to the development of drug resistance [44].