A team led by Dr David Glahn (Glahn et al., 2012) claims to have identified a gene RNF123 which may play a role in major depression (as distinct from bipolar disorder/depression). This gene has been shown to affect the hippocampus, which in turn is implicated in depression. Smaller hippocampal volumes are often found in people with recurrent bouts of major depression. Smaller hippocampal volumes also appear to be associated with a lower probability of remission of depression with antidepressants.However, the causal nature of this relationship is not entirely clear. Hippocampal volume may be either a cause of depression or a consequence of it. For example, it might be the case that people born with a smaller hippocampus (because of the RNF123 gene) may be more vulnerable to depression (the diathesis-stress model). Alternatively, it could also be the case that the duration of untreated depression might in some way affect hippocampal volumes.
The picture above is apparently what RNF123 looks like!