EGFR and lung carcinoma: For example, during the course of gefitinib treatment for EGFR‐mutant lung cancers, a secondary mutation could lead to the conversion of threonine to methionine at the amino acid position 790 in the exon 20 of the EGFR gene, resulting in an increase in the spatial structure of side chains of the target site to enhance steric hindrance to the therapeutic drug.