FOXRED1 and hyperinsulinemic hypoglycemia, familial, 4: Overall, in this work we describe two cases of complex I deficiency due to FOXRED1 pathogenic variants and discuss how FOXRED1 variants may cause a broad clinical spectrum and be associated with different levels of disease severity—from the severe clinical presentation of patient 6, a three-month-old girl who died from disease complications to the mild presentation of patient 2, the brother of our index case.