The BTBR T+ Itpr3tf/J (BTBR) inbred strain has increased susceptibility to obesity and diabetes compared to the C57BL/6J (B6) strain. These effects are exaggerated in the presence of an obesogenic stimulus, e.g. homozygosity for the obese allele of the leptin gene(Lepob). Prior genetic studies mapped the loci affecting body weight and metabolic traits (fasting glucose and insulin) segregating between these strains. These studies identified the modifier of obesity 1 (Moo1) locus on mouse Chromosome 2 as the major locus controlling the difference in adiposity between these strains in Lepob/ob mice. Congenic strains initially identified a ~6 Mb region of Chromosome 2 where replacement of BTBR alleles with those from B6 reduces body weight ~10% (the Moo1-C strain). Work by my laboratory has shown that this locus also affects obesity induced by high fat feeding, and is associated with alterations in body fat, food intake and glucose tolerance. We have also detected an interaction between this locus and stress that affects body weight. To localize the causative genetic variation we created a panel of 10 sub-congenic strains from Moo1-C strain recombinants. Analysis of these strains revealed Moo1 is comprised of at least 2 independent QTLs. The proximal of these, Moo1a, is encompassed within a strain (Moo1-V) in which B6 alleles replace only 319 kb of BTBR genome (rs27970625 – D2Mit328). Homozygosity for B6 alleles in this region reduces body weight of high fat fed BTBR mice by ~8 % (P<0.001). This region spans 2 known genes: Itga6 and Pdk1. We identified numerous (non-coding) sequence changes within and near these genes and each gene has a single coding variation between the strains, although these are not predicted to affect protein function. We assessed expression of these genes in metabolically relevant tissues and found an ~50% reduction in expression of both Itga6 and Pdk1 in many tissues. Pdk1 regulates the entry of glucose-derived metabolites into the Krebs cycle for oxidation. This also regulates substrate availability for the synthesis of fat, making this an obvious candidate. We obtained mice deficient in PDK1 from the KOMP repository, but found no evidence of alterations in obesity in high fat-fed heterozygous or homozygous knockouts compared to their wildtype littermates. Itga6, although the less obvious candidate, is an integrin that could affect obesity in several ways. Analysis of mice with reduced ITGA6 is ongoing. These data highlight the power of mouse genetics to discover novel molecular pathways affecting complex metabolic phenotypes that are highly influenced by environmental factors and to identify specific gene-environment interactions.