C3 and obesity due to melanocortin 4 receptor deficiency: Factor I (F-I) cleaves C3b, leading to the degradation products C3b and iC3b, which is then unable to bind F-B [26]; F-I was elevated in both obese and nonobese PCOS and associated with BMI in obese PCOS, suggesting contributions in its elevation from both the inherent PCOS phenotype and obesity.