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Figure 2 Retrograde conduction in mutant transgenic mice. Electrocardiographic leads I and II (LI and LII) are shown with right ventricular (RV EG) and right atrial electrograms (RA EG). Ventricular pacing at 115 ms in the presence of procainamide, with retrograde conduction up the bypass tract for the first eight beats and fusion of atrial (A) and ventricular (V) electrograms on both the RV EG and RA EG. The ninth beat (arrow) and subsequent beats conduct with a longer RP interval, revealing retrograde block in the bypass tract and conduction up the AV node. Note there is now separation of the A and V electrograms on both intracardiac electrograms.