A deletion results in an absence of the enzyme, leading to reduced capacity for hepatic detoxification and increases the likelihood that mutagenic DNA adducts will be produced. The GSTM1 null genotype is associated with an increased risk of various cancers, chemical sensitivity, coronary artery disease in smokers, atopic asthma, and deficits in lung function.
If you have a deletion in this gene, then it is recommended that you minimize your exposure to toxins and pollutants such as cigarette smoke, pesticides, PAHs, acrylamides, etc. Ensure the diet is rich in antioxidants and substantially increase intake of cruciferous and allium vegetables to increase the activity of other GST enzymes. Adding in bioavailable glutathione (not just the amino acid precursors, namely cysteine, glutamic acid, and glycine) is important for upregulating this pathway.
Other nutrients/minerals that may increase the production of glutathione include vitamin B6, magnesium, selenium, folate (B9), and alpha-lipoic acid.