(A). MKK4cko-TAC mice are more susceptible to ventricular arrhythmias and slowed conduction velocity. (B). Most activation maps from MKK4F/F hearts showed a sequential activation pattern, beginning with a localized activation of one region of the matrix followed by its successive spread to the remaining area from which recordings were made. However, the old MKK4CKO hearts often showed a more disordered excitation sequence.