The present invention relates generally to the field of human genetics. Specifically, the present invention relates to methods and materials used to isolate and detect a human obesity and diabetes predisposing gene, specifically the TBC1D1 gene, some mutant alleles of which cause susceptibility to obesity and/or diabetes. More specifically, the invention relates to germline mutations in the TBC1D1 gene and their use in the diagnosis of predisposition to obesity and diabetes. Finally, the invention relates to the screening of the TBC1D1 gene for mutations/alterations, which are useful for diagnosing the predisposition to obesity.