Inhibition of EGFR can result in dose-limiting rashes and gastrointestinal issues, particularly diarrhea to the level of Grade 3 or 4 toxicity (as observed in the LUX-BREAST-1/−3 studies of afatinib [4, 5] and the ExteNET trial of neratinib [6] in HER2-positive breast cancer). The gene discussed is ERBB2; the disease is breast cancer.