in subsequent association analysis [64, 65]. In addition, a number of other candidate genes have been identified via association studies, including the genes encoding cholinergic receptor nicotinic alpha 4 (CHRNA4; 20q13.2-q13.3), gamma-aminobutyric acid A receptor gamma 2 (GABRG2; 5q31.1-q33.1) and -beta 3 (GABRB3; 15q11.2-q12), interleukin 1 beta (IL1B; 2q14) and interleukin 1 receptor antagonist (IL1RN; 2q14.2) [66-70]. However, the latter associations could not be replicated in subsequent cohorts [71-75].
