GSTM1 and periodontitis: Patients were more susceptible to chronic periodontitis if: both the GSTMI and T1 were absent (p < 0.001); they were GSTM1 null and had any genotype at GSTP1 (p < 0.001); they were GSTT1 null with heterozygous (AG) and mutant (GG) genotype (p < 0.001); or they had GSTT1 present but GSTM1 absent with any genotype at rs1695 in GSTP1 (p < 0.001) (Table 3).