Updated FTP location. An ultrasensitive quantitative phosphoproteomic approach to measure the dynamic phosphorylation stoichiometry in oncoreceptor, its interaction proteins and downstream cellular proteins. Using epidermal growth factor receptor (EGFR) as a model, the γ-[18O4]-ATP kinase strategy greatly enhanced sensitivity to obtain 16 pY sites on EGFR in a single experiment and 4%-34% higher pathway mapping coverage.