To determine parameter estimates for individual CYP2A6 haplotypes, further regression analyses were performed assuming a multiplicative effect of haplotype. Two subjects carrying rare non-synonymous polymorphisms were excluded from this analysis (see results). Two versions of this regression were performed. First, β parameters for haplotypes CYP2A6*2 and CYP2A6*4 were fixed at one (zero activity) and parameter estimates were obtained for all other haplotypes. Linear hypothesis tests were then performed on the resulting model. These were used to determine which haplotype effects are significantly different from the most common haplotype (*1B), used here as the reference haplotype. Based on these results, for the final analysis β parameters for haplotypes CYP2A6*2, CYP2A6*4, CYP2A6*12, and CYP2A6*1D-Y351H were fixed at one and the remaining haplotypes were grouped into equivalence classes corresponding to those determined significantly different and readily distinguished by genotype. In particular, three haplotype parameters were modeled: the first for CYP2A*9, the second for CYP2A6*1A(51A) and CYP2A6*1B12, and the third for the remaining six haplotypes (*1B, *1B7, *1A(51G), *1H, *1D and *1X2). To display parameter estimates intuitively in terms of relative activities in tables
