INS and obesity due to melanocortin 4 receptor deficiency: Adp1n may explain why R0 consomic mice in our previous study were insulin resistant without body weight gain, in that insulin resistance and adiposity were controlled by Nidd2.1n, while adiposity was reduced and obesity was masked by the effect of Adp1n. Mice with NSY alleles at both Nidd2.1n and Adp1n, as in the case of R0 and R1, show insulin resistance without body weight gain, while mice with the NSY allele at Nidd2.1n, but with the C3H allele at Adp1n, as in the case of R2, show insulin resistance with obesity.