When we analyzed the combined effect of GSTM1 and GSTT1 null polymorphisms, we found a significant prevalence of at least one null genotype (either GSTM1 or GSTT1 or both null genotypes simultaneously) in COPD patients compared to controls (p = 0.027, Table 3). This evidence concerns the gene GSTM1 and chronic obstructive pulmonary disease.