Erlotinib is an inhibitor of human Epidermal growth factor receptor (EGFR) tyrosine kinase (IC₅₀ = 2 nM) and decreases EGFR autophosphorylation in tumor cells (IC₅₀ = 20 nM). Studies in cell lines and enzyme assays have both shown that erlotinib inhibits EGFR at concentrations significantly lower than those needed to inhibit c-src and v-abl.