- Ethnicity; European and African American will be selected for study. Other minorities will not be studied owing to the problem of population stratification, which refers to differences in allele frequency between cases and controls that are due to differences in ancestry rather than associations of genes with disease or drug response (47). For European Americans, the frequency of Arg16 homozygosity is about 15% compared to about 30% in African Americans (48). In previous studies, the PI restricted his analyses to European Americans because of the potential confounding of population stratification (48;49). .