Molecular genetics has now been widely incorporated into genetic epidemiological research in psychiatry. Affective disorders and, in particular, bipolar affective disorder (BPAD) have been examined in many molecular genetic studies which have covered a large part of the genome. Specific hypotheses such as mutations have also been studied. Most recent studies indicate that several chromosomal regions may be involved in the aetiology of BPAD. These include genes on chromosomes 18, 21, 4, 5, 11 and X. Other studies have reported the presence of anticipation in BPAD and in unipolar affective disorder (UPAD). This phenomenon describes the increase in clinical severity and decrease in age of onset observed in successive generations. This mode of transmission correlates with the presence of specific mutations (trinucleotide repeat sequences). Associations with these mutations have been reported in different populations of BPAD-patients and may represent a genetic factor involved in the transmission of the disorder.

These findings are all preliminary and require to be confirmed. Large multi-centres and multi-disciplinary projects are currently underway in Europe and in the US and hopefully will improve our understanding of the genetic factors involved in affective disorders. In addition, genetic approaches used in psychiatry are being combined with an assessment of non-genetic susceptibility factors. The investigation of interactions between gene and environment is one of the most promising areas dealing with complex multi-factorial diseases such as the affective disorders.