The If current (funny current) is involved in the pacemaker mechanism of cardiac cells. In the human sinus node, If is mainly encoded by the HCN4 gene. Less than 10 pathogenic mutations of this gene were found to date, all related to bradycardia [1].Two novel heterozygous mutations were identified in Chinese patients with lone atrial fibrillation and bradycardia, and were absent in 200 ethnically matched control subjects. One, (D600E) is located near the CNBD site, and the other (V501M) in the pore domain. We sought for possible mutation-induced functional changes in HCN4 channels.Cos7 cells were transfected with either 3 µg (per 35 mm Petri dish) wild-type (WT) HCN4 cDNA plasmid, or with 3 µg mutated HCN4 cDNA plasmid or with a mixture of 1.5 µg of each. Cotransfection with a CD8 surface antigen plasmid allowed detection of transfected cells. Whole-cell currents were recorded with the patch-clamp technique, at room temperature.Currents measured at -130 mV in Cos7 cells expressing D600E alone (-19.2±3.5 pA/pF, n=22) were larger (p<0.05, unpaired Student’s t-test) than in cells expressing the WT (-11.9±2.1 pA/pF, n=30). This difference was abolished in cells expressing D600E+WT. The mutant, alone or in coexpression with the WT, did not change the kinetics of activation. However, deactivation was accelerated 1.6 fold versus WT in cells expressing either D600E alone or D600E+WT.Cells transfected with V501M alone did not show any HCN4 current. In cells expressing V501M+WT, the current (-11.7±5.6 pA/pF, n=5) was similar to that of WT (-11.9±2.1 pA/pF, n=30). Neither V501M alone nor V501M+WT changed the time constants of activation and deactivation. However, V501M+WT caused a negative shift by 9 mV of the voltage for half steady-state activation (-104.9±3.4 mV vs -96.1±1.9 mV for WT). The reversal voltage of the HCN4 current was not changed by mutants alone or coexpressed with the WT.The impact of each mutant in the heterozygous condition was explored in a computer model of sino-atrial node cell [2] run at steady-state in spontaneous mode. When deactivation of the If current was accelerated 1.6-fold (D600E+WT effect), the If current amplitude and the slope of the slow diastolic depolarisation were lowered and interval between action potentials was lengthened from 332 to 352 ms (+6%). Alternatively, introducing a -9 mV shift of the voltage of half-activation (V501M+WT effect) also reduced If current amplitude and the slope of diastolic depolarisation, resulting in a cycle length of 412 ms (+24%).Through accelerated deactivation of the If current or a negative shift of steady-state activation each mutant may cause bradycardia, a condition favouring atrial fibrillation. Thus, these HCN4 mutants likely contribute to atrial fibrillation by different mechanisms.