Exon 19 deletion mutations account for 45% of mutations, whereas Exon 21 mutations, resulting in L858R substitutions, account for 40% to 45% of mutations that predict sensitivity to EGFR TKIs (like erlotinib, afatinib, gefitinib, and osimertinib) to slow or halt cancer cell growth. This evidence concerns the gene EGFR and cancer.