Similar to Bartter syndrome type IV, in which the auditory phenotype is only observed when Barttin, the subunit for both ClC-K chloride channels (ClC-Ka and ClC-Kb), is genetically impaired (44), mutations in VAPB might be responsible for cardiac arrhythmias that were not found for HCN1 or HCN2 channels alone. Here, HCN1 is linked to chronic obstructive pulmonary disease.