CYBB and hydrops fetalis: In HF, chronic oxidative stress is linked both to an elevation in ROS levels due to the dysfunctional electron transport system (ETS) and to an increase in the activity of several enzymes such as NADPH oxidases 2 and 4 (Nox2 and Nox4, the latter controversially), monoamine oxidase isoform A (MAO-A), uncoupled mitochondrial NO synthase (mtNOS) and a reduction of the antioxidant defense systems (e.g., mitochondrial superoxide dismutase/mt SOD).