PER2 and Behcet disease: Indeed, 3 studies have previously found associations of NPAS2 gene with BD34, 35, 36, and a winter SP with either MDD or BD populations being associated with: i) an amino acid substitution in NPAS2 (471 Leu/Ser)24; and ii) a combination of variations in 3 circadian genes that include NPAS2 (PER2, ARNTL, and NPAS2)25.