**1. Introduction**

Hypoplastic left heart syndrome (HLHS) is a complex congenital heart disease (CHD) involving hypoplasia of the left ventricle (LV), aorta (Ao), and mitral valve (MV). While HLHS is relatively rare (prevalence 0.02%) [1], it accounts for 25% of CHD infant deaths [2]. The great surgical advancement in HLHS led to a transition from the only option of comfort care in the 1980s to a three-stage cardiac surgical palliation procedure offering a 50–70% five-year survival [2]. However, about 25% of HLHS patients either died or had a heart transplant within 1 year of their Norwood operation [3]. Most HLHS survivors suffer ongoing morbidity, with a substantial portion of these patients developing heart failure over time, as well as neurodevelopmental delay and neurocognitive impairment that can significantly degrade the

health-related quality of life. The causes for such high morbidity and mortality are not well understood, but the majority of deaths are directly or indirectly related to cardiovascular causes [4]. While the causes for the poor cardiac outcomes are multifactorial, our study in a mouse model of HLHS uncovers important contributing factors related to mitochondrial dysfunction [5].
