A study of the association of circadian genes with mood disorders [21] found that while two specific genotypes (rs10462028 in the circadian locomotor output cycles kaput (CloCK) transcription factor gene and rs17083008 in the vasoactive intestinal peptide (VIP)) were associated with BD, there was a reduced susceptibility for BD with the subjects heterozygous for rs10462028 in CloCK and rs17083008 in VIP; this is typical for a mutation that could yield benefits in heterozygous populations but be catastrophic in homozygous ones. This evidence concerns the gene VIP and Behcet disease.