With the S1S2 pulse protocol, the incidence of ectopic deflections was similar between groups, but the incidence of atrial tachycardias was greater in aged Pgc1β−/− hearts than in young WT, aged WT or young Pgc1β−/− hearts, which all showed similar incidences of atrial tachycardias (P = 0.043). The gene discussed is PPARGC1B; the disease is atrial tachycardia.