LY75 and obesity due to melanocortin 4 receptor deficiency: In the present study, we obtained genetic evidence that Ly75 is a true QTG for the Pbwg1.5 QTL for resistance to obesity by using a combined approach of a quantitative complementation test, qualitative phenotypic analyses and causal analysis in segregating populations obtained from crosses using the B6.129P-Ly75tm/Mnz/J strain knocked out for Ly75 (hereafter called KO)13, the B6.Cg-Pbwg1/24Nga subcongenic strain carrying Pbwg1.5 (SR24; Supplementary Fig. S1) and their background strains of C57BL/6 J (B6J) and B6JJcl.