The present invention provides a method of diagnosing bipolar affective disorder (BAD) and for determining a predisposition of a subject to bipolar affective disorder. In particular, the methods of the present invention comprise detecting a marker that comprises one or more polymorphisms at position 4q35.2 of the human genome and/or one or more allelic variants of the FAT gene associated with BAD or linked thereto. The present invention also relates to identifying new markers that are diagnostic of bipolar affective disorder. Furthermore, the present invention relates to methods of identifying and producing candidate compounds for the treatment of bipolar affective disorder.