Because 12 of the 13 types of rare variants observed were located in exon 5 of CHRNA4, including all of the rare nonsynonymous variants, we sequenced this exon in an additional 1000 ND cases and 1000 comparison subjects. Per Uniprot, exon 5 encodes 114 amino acids of the extracellular domain, all of the first three transmembrane domains, and 256 amino acids of the cytoplasmic loop.
