FH and hyperinsulinemic hypoglycemia, familial, 4: Notably, FHi led to a more pronounced decrease in Complex I biogenesis and subunits (Figure 5—figure supplement 1C) consistent with the previous reports on FH-deficiency regulating iron-sulphur cluster biogenesis and complex I activity (Tong et al., 2011; Tyrakis et al., 2017), while also explaining the noticeable differences in the NADH/NAD+ ratio between FHi and SDHi (Figure 4E).