Cardiac and skeletal muscle cells, when relaxed, have fewer positively charged ions on the inner side of their cell membrane than on the outer side, referred to as their membranes being polarised. The main ion current responsible for maintaining this polarity is /K1, and a decrease in this current leads to less polarity at rest, or a depolarised resting membrane potential. When these cells contract, positively charged ions such as sodium and calcium enter the cell through ion channels, depolarising or reversing this polarity. After a contraction has taken place, the cell restores its polarity (or repolarises) by allowing positively charged ions such as potassium to leave the cell, restoring the membrane to its relaxed, polarised state. The genetic mutation found in those with Andersen–Tawil decreases the flow of potassium, slowing the rate of repolarisation which can be seen in individual cardiac muscle cells as a longer action potential and on the surface ECG as a prolonged QT interval.