MEK inhibitors are associated with a high frequency of mucocutaneous toxicities (rash, xerosis, dermatitis, paronychia), gastrointestinal events (diarrhea, nausea, colitis), ocular toxicities (blurred vision, MEK inhibitor-associated retinopathy), and cardiovascular effects (hypertension, decreased ejection fraction, QTc prolongation), with most adverse events being grade 1–2 but grade ≥ 3 events requiring dose reduction or interruption in a significant minority of patients. The gene discussed is MAP2K7; the disease is paronychia.