study them in meaningful subgroups. Dual users have distinct clinical features that distinguish them from those who are exclusive users of crack or snorted cocaine [13]. In the present study, allele frequencies for rs1803274 were very similar to the values for the Haplotype Map population of Utah residents with ancestry from northern and western Europe (CEU population). We also found that the frequency of the AA genotype in our sample as a whole was greater than in the CEU population (3.3% compared to 1.7%). The other two markers investigated have been infrequently used in association studies. Twelve BCHE SNPs were tested in farmers exposed to organophosphates and rs4680663 was not found to be associated with cholinesterase activity [36]. We found no association studies using the SNP rs4263329 as a marker.
