Inward rectifier currents carried by K IR 2.1 proteins have an important role in cardiac electrophysiology. Animal knock-outs and human loss-of-function mutation carriers experience cardiac pro-arrhythmia, but phenotypes are not confined to the heart since these channels are prominently expressed in many other organs and tissues. We here review the other end of the spectrum, in which gainof-function of the K IR 2.1 carried I K1 results in action potential shortening in isolated cardiomyocytes, and QT shortening in animals and humans. Gain-of-function mutations in patients often result in short QT syndrome accompanied with atrial fibrillation. Remarkable, skeletal muscle, neurological and developmental abnormalities are less prominent in these patients compared to their loss-of-function counterparts. Finally, the most common pathological arrhythmia, atrial fibrillation, is associated with K IR 2.1 upregulation at the mRNA and protein level, and concomitant enhanced I K1 density in atrial tissues.