Although male Mecp2Null/Y mice recapitulate the phenotypes seen in Mecp2Null/+ at an earlier age (McCauley et al., 2011), the Mecp2Null/+ model is more physiologically relevant to the disorder because affected RTT individuals are nearly always girls and women with heterozygous mutations in MECP2. Mecp2Null/+ mice show a progressive onset and worsening of cardiac phenotypes such as the development of LQT and arrhythmias (McCauley et al., 2011) similar to the developmental regression observed in people with RTT (Neul et al., 2014). This evidence concerns the gene MECP2 and cardiac arrhythmia.