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 depression predisposing gene, specifically the apoptotic protease activating factor 1 (APAF1) gene, some mutant alleles of which cause susceptibility to depression. More specifically, the invention relates to germline mutations in the APAF1 gene and their use in the diagnosis of predisposition to depression. The invention also relates to the prophylaxis and/or therapy of depression associated with a mutation in the APAF1 gene. The invention further relates to the screening of drugs for depression therapy. Finally, the invention relates to the screening of the APAF1 gene for mutations/alterations, which are useful for diagnosing the predisposition to depression.