As nAChR subunit gene clusters on chromosomes 15 (CHRNA5/A3/B4) and 8 (CHRNB3/A6) are major discoveries from ND GWAS, their candidate association results will be discussed together with the GWAS results. Significant association of variants in two other subunit genes (CHRNA4 and CHRNB1) did not approach genome-wide significance (p < 5 × 10−8), but they are both close to nominated linkage peaks.34 Association of CHRNA4 with ND, close to the nominated linkage peak on 20q13.12–13.32,34 has been demonstrated in five independent studies (Table 2).90, 97-100 Variants within CHRNB1, located close to the nominated linkage peak on 17p13.1-q22,34 are significantly associated with FTND and CPD scores.90, 101 Two other genes encoding nAChR subunits, CHRNB2 and CHRNA2, although associated with ND-related phenotypes in two studies,102, 103 are not within any detected linkage peaks and have no replication studies reported that are of the required sample size. Thus, these two genes are considered to have only weak evidence of involvement in ND and therefore are not included in Figure 1 and Table 2. Besides nAChR subunit genes, two cholinergic receptors, muscarinic 1 (CHRM1) and
