HCN1 and Bartter syndrome type 4: 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.