organs including the brain. This results in organ damage and strokes, severely shortening the lifespan of the individual. Nonetheless, the HbS allele is maintained in the gene pool in regions where malaria is endemic because in heterozygous form it provides protection against malaria (for a review see [6]. The case of nicotine addiction represents a similar conundrum. Several genetic variants that modify susceptibility to or protection from nicotine dependence have been identified by genome-wide association studies (GWAS) [2,4,7]. Perhaps not surprisingly, the loci identified in these studies mainly include genes encoding neuronal nicotinic cholinergic receptors (CHRNs).
