This study has several unique strengths. First, this is a family-based association study, consisting of one of the largest cohorts recruited for studying the genetics of ND. Second, our samples include two major ethnicities in the USA, African-Americans and European-Americans, permitting an examination of potential genetic differences underlying the expression of ND. Finally, as this is a family-based association study, it is considered to be robust to population stratification and thus reduces the likelihood of false association caused by sample admixture. However, although the family-based association study can minimize the potential impact of population stratification or admixture on association results, it could not completely eliminate such an effect. Thus, more caution is needed when one uses samples comprised of participants with different ethnicity for an association study.
