INS and obesity due to melanocortin 4 receptor deficiency: However, when investigating at the characterization of the MC4R knock-out mice, Cone, et al. found that animals heterozygous for the melanocortin-4 receptor were intermediate to wild type and MC4R null mice in obesity-related phenotypes including growth rate, serum leptin concentrations, linear growth, and fasting serum insulin concentrations [45].