Nonsmokers who carry HLA DRB1*15:01 and lack HLA A*02 have a combined OR of ~5; however, among smokers, the combined OR is ~14, much higher than the sum of the main effects associated with each factor.117 Smoking may also interact with the NAT1 gene (encoding N‐acetyltransferase 1 enzyme that is important in the metabolism of aromatic amines present in cigarette smoke) to alter MS risk.118. This evidence concerns the gene HLA-DRB1 and myeloid sarcoma.