Two genes exceeded genome-wide significance with smoking cessation: the SLC25A21 gene on chromosome 14 and with the SEMA6D gene on chromosome 15 (P=1.64E-06). The role these genes play in smoking cessation and their potential as therapeutic targets has yet to be elucidated. SEMA6D expression levels were recently shown to significantly predict survival in patients with breast cancer 36; also the gene was listed among potential targets in cancer therapy owing to its role in regulation of the immune response 37. The SLC25A21 gene was previously proposed as a plausible candidate for smoking cessation 38 and current smoking 39. Notice that the SLC25A21 gene was the only one significantly associated with two smoking behaviours, namely with quantity smoked (P=1.41E-05) and smoking cessation (P=2.09E-08), result consistent with the large genetic correlation between the two smoking behaviours (rg = -.62; P<.001) 40 observed in this sample.
