Fifty-five studies had a quality score ≥ 6 and the remaining 31 had a quality score < 6 regarding the GSTM1 present/null polymorphism; 48 high-quality studies were examined and the remaining 16 were low-quality concerning the GSTT1 present/null polymorphism; a total of 25 high-quality and eight low-quality studies were included on their combined effects with CRC risk. The gene discussed is GSTM1; the disease is colorectal carcinoma.