['Atrial fibrillation (AF) is the most common arrhythmia, and a recent genome-wide association study identified the hyperpolarization-activated cyclic nucleotide-gated channel 4 (HCN4) as a novel AF susceptibility locus. HCN4 encodes for the cardiac pacemaker channel, and HCN4 mutations are associated with familial sinus bradycardia and AF.', 'The purpose of this study was to determine whether novel variants in the coding region of HCN4 contribute to the susceptibility for AF.', 'We sequenced the coding region of HCN4 for novel variants from 527 cases with early-onset AF from the Massachusetts General Hospital AF Study and 443 referents from the Framingham Heart Study. We used site-directed mutagenesis, cellular electrophysiology, immunocytochemistry, and confocal microscopy to functionally characterize novel variants.', 'We found the frequency of novel coding HCN4 variants was 2-fold greater for individuals with AF (7 variants) compared to the referents (3 variants). We determined that of the 7 novel HCN4 variants in our AF cases, 1 (p.Pro257Ser, located in the amino-terminus adjacent to the first transmembrane spanning domain) did not traffic to cell membrane, whereas the remaining 6 were not functionally different from wild type. In addition, the 3 novel variants in our referents did not alter function compared to wild-type. Coexpression studies showed that the p.Pro257Ser mutant channel failed to colocalize with the wild-type HCN4 channel on the cell membrane.'] Is a novel trafficking-defective HCN4 mutation associated with early-onset atrial fibrillation?