Mutations in SCN4A that enhance the function of NaV1.4 are found in patients with myotonia (delayed muscle relaxation after contraction) due to spontaneous repetitive action potential firing – manifesting clinically as muscle stiffness or cramp – and/or periodic paralysis (PP) that presents as episodic muscle weakness or paralysis1–4. The gene discussed is SCN4A; the disease is periodic paralysis.