an association of the region with ND in European American and African American samples [18]. This has been the case for several other genes also. For example, we recently detected a significant interaction between variants of CHRNA4 and CHRNB2 in affecting ND, such that CHRNB2 contributes significantly to the etiology of ND together with CHRNA4 through gene-gene interaction [34]. Similarly, in another study, we found that GABAB1 contributes to ND through its interaction with GABAB2, although no significant association was observed for GABAB1 with ND directly [35].
