IGF2 and obesity due to melanocortin 4 receptor deficiency: Interestingly, Igf2 and Peg3, which are paternally expressed imprinted genes involved in the regulation of body fat accumulation, were down-regulated in B6 and (PWK×B6) F1 mice, which are susceptible to HFD-induced obesity, but not in PWK and (B6×PWK) F1 mice, which are resistant.