MDM2 and obesity due to melanocortin 4 receptor deficiency: We focused on the MDM2-p53 axis as a potential regulator of C3 because: (a) MDM2 is the primary ubiquitin ligase responsible for degradation of the guardian of the genome p53 protein (56), (b) p53 has been previously shown to promote obesity (44, 44, 46), and (c) and several putative p53 consensus binding sites are located in the c3 gene promoter (EPD and JASPAR databases).