Gu J, Zou H, Zheng L, Li X, Chen S, Zhang L. GSTM1 null genotype is associated with increased risk of gastric cancer in both ever-smokers and non-smokers: a meta-analysis of case-control studies. Tumour Biol. 2014 Apr;35(4):3439–45. doi: 10.1007/s13277-013-1454-z. PMID: 24318990.