KCNH2 (hERG1) encodes the α-subunit proteins for the rapidly activating delayed rectifier K⁺ current (I_Kr), a major K⁺ current for cardiac myocyte repolarization. In isolated myocytes I_Kr frequently is small in amplitude or absent, yet KCNH2 channels and I_Kr are targets for drug block or mutations to cause long QT syndrome. We hypothesized that KCNH2 channels and I_Kr are uniquely sensitive to enzymatic damage. To test this hypothesis, we studied heterologously expressed K⁺, Na⁺, and L-type Ca²⁺ channels, and in ventricular myoctyes I_Kr, slowly activating delayed rectifier K⁺ current (I_Ks), and inward rectifier K⁺ current (I_K1), by using electrophysiological and biochemical methods. 1) Specific exogenous serine proteases (protease XIV, XXIV, or proteinase K) selectively degraded KCNH2 current (I_KCNH2) and its mature channel protein without damaging cell integrity and with minimal effects on the other channel currents; 2) immature KCNH2 channel protein remained intact; 3) smaller molecular mass KCNH2 degradation products appeared; 4) protease XXIV selectively abolished I_Kr; and 5) reculturing HEK-293 cells after protease exposure resulted in the gradual recovery of I_KCNH2 and its mature channel protein over several hours. Thus the channel protein for I_KCNH2 and I_Kr is uniquely sensitive to proteolysis. Analysis of the degradation products suggests selective proteolysis within the S5-pore extracellular linker, which is structurally unique among Kv channels. These data provide 1) a new mechanism to account for low I_Kr density in some isolated myocytes, 2) evidence that most complexly glycosylated KCNH2 channel protein is in the plasma membrane, and 3) new insight into the rate of biogenesis of KCNH2 channel protein within cells.