To search for rare variants likely to influence function in CHRNA4 we analyzed whole-genome sequence data from 2636 Icelandic subjects, identifying eight non-synonymous variants present in high enough frequency to allow imputation into our long-range phased data set21 (usually approximately five carriers in our sequence data, currently corresponding to an allele frequency of about 0.1%), generating genotypes for 104 220 chip-typed Icelanders and 294 212 close relatives. Eight variants met our criteria, and following imputation they were tested for association with the FTND score (Table 1) with the significance threshold set at P=6.25 × 10−3 (0.05/8). The mutation encoding R336C associates with FTND score (P=1.2 × 10−4), whereas none of the other mutations tested showed significant association (Table 1). To improve imputation accuracy, we typed 1441 subjects for the R336C variant using a single-marker assay (Nanogen) and repeated the imputation process. As a result of this effort the imputation information increased from 0.939 to 0.999, and all results reported here were based on re-imputed data. On the basis of 1408 (original imputation) or 1441 individuals (re-imputation) with both typed and
