GSTM1 and breast cancer: Homozygous deletion of GSTM1 gene was more frequent in the BC patients than in controls (61.1 vs. 45.2 %, P < 0.0001), while homozygous deletion of GSTT1 gene was more rarely observed in the BC patients than in controls (12.4 vs. 21.1 %, P = 0.006).