To further advance our understanding of EGFR regulation, we immunoprecipitated endogenous EGFR from whole-cell extracts of A431 human cancer cells, which had been serum starved overnight and then stimulated for 5 min with EGF, and subjected trypsin digests of EGFR-associated proteins to liquid chromatography-tandem mass spectrometry (LC-MS/MS). The gene discussed is EGF; the disease is cancer.