Of the included 8 studies, check details one was written in French [13], three in Chinese [8, 9, 12] and the remaining four studies [7, 10, 11, 14] were written in English. The controls of the included studies are in agreement with Hardy-Weinberg equilibrium. We established

a database according to the extracted information from each article. The information was listed in Tab. 1. According to the lists, the first author and the number of cases and controls for each study as well as other necessary information were GSK461364 presented. Table 1 Case-control studies on GSTM1/GSTT1 polymorphisms and NPC risk First Author Publication Year Cases Controls Histology Ethnicity genotype Ref. number Nazar-Stewart V 1999 83 142 11 Epithelial, Nos; 24 Undifferentiated; 48 Squamous 57 Caucasian; 7 African-American; 17 Asian; 2 Native American GSTM1 [7] Da SJ 2002 80 80 72 Squamous, 8 Adenocarcinoma 80 Asian (China) GSTM1 [8] Cheng YJ 2003 314 337 Not Determined 314 Asian (China) GSTM1; GSTT1 [11] Deng ZL 2004 91 135 91 Squamous 91 Asian (China) GSTM1; GSTT1 [12] Liao ZL 2005 80 72 Not Determined 80 Asian (China) GSTM1 [9] Tiwawech D 2005 78 145 Not Determined 78 Asian (Thailand) GSTM1 [10] Bendjemana

K 2006 45 100 Not Determined 45 Caucasian (France) GSTM1; GSTT1 [13] Guo X 2008 341 590 Not Determined 341 Asian (China) GSTM1; GSTT1 [14] Figure 1 The flow diagram of included/excluded studies. Test of heterogeneity Fig. 2 shows the association between the GSTM1 deletion and NPC risk. We analyzed the heterogeneity for all 8 studies and the test value of Chi-square was 6.73 CHIR98014 nmr with 7 degree of freedom (d.f.) and P > 0.05 in a fixed-effect model. For the association between the GSTT1 null genotype and NPC risk, the Chi-square value for the heterogeneity of all 4 studies was 7.16 with 3 d.f. and P > 0.05 in a fixed-effect

model (Fig. 3). Figure 2 Meta-analysis with a fixed-effect model for the association of NPC risk with GSTM1 polymorphism (null genotype versus present genotype). Figure 3 Meta-analysis with a fixed-effect model for the association Acyl CoA dehydrogenase of NPC risk with GSTT1 polymorphism (null genotype versus present genotype). Additionally, I-square value is another index for the heterogeneity test [15], with value less than 25% indicating low, 25% to 50% indicating moderate, and greater than 50% indicating high heterogeneity. In Fig. 2, the I-square value was 0%, suggesting an absence of heterogeneity. Thus, a fixed-effect model was used. However, in Fig. 3, the I-square value was 58.1%, suggesting a possible presence of heterogeneity. Accordingly, both fixed-effect model (Fig. 3) and random-effect model (Fig. 4) were utilized for evaluation of GSTT1. Figure 4 Meta-analysis with a random-effect model for the association between NPC risk and the GSTT1 polymorphism (null genotype versus present genotype).