The exact mechanism by which MEK inhibitors cause hyponatremia remains unknown, although it is theorized that BRAF/MEK inhibitors lead to hyponatremia and water retention (dilutional hyponatremia) by activating aquaporin 2 (AQP2) trafficking from its intracellular compartment to the apical cell membrane of the renal tubule.9,12 In the renal collecting ducts, vasopressin regulates water permeability through AQP2. The gene discussed is AQP2; the disease is Hyponatremia.