Gel electrophoresis showing PCR products of GSTT1, GSTM1 and b-globin. L; Molecular weight marker. N; Negative control. T+/M+; DNA from patients with positive GSTM1, GSTT1, and β-globin alleles. T-/M-; Double null genotype of GSTM1 and GSTT1 (in the presence of β-globin PCR product). T+/M-; GSTT1 null/GSTM1 positive. T-/M+; GSTT1 positive/ GSTM1 null.