The human ether-a-go-go related gene (hERG; officially designated as KCNH2) encodes the structure of protein forming alpha-subunit of voltage-gated ion channel which conducts the rapid component of delayed rectifier K+ current (IKr). This current plays an important role namely in the cardiac repolarization. Mutations in hERG result in inherited arrhythmogenic syndromes characterized by a lenghtening or shortening of QT interval on the electrocardiogram and by an increased occurrence of life-threatening arrhythmias. This review also introduces hERG channels as a part of regulatory mechanisms of the smooth muscle contractility, neuronal activity, release of several hormones, and of proliferation and apoptosis of cancer cells. There are also mentioned some of the diseases arising from hERG channel dysfunction, and some possibilities of use of hERG gene/channel as a diagnostic marker and potential therapeutic target in various diseases, namely in cancer.