MMP9 and lung carcinoma: Compared with several other meta-analysis about MMP polymorphisms and lung cancer risk reported by Guo XT et al45, Hu J et al 46 and Wang J et al 47, we identified more eligible studies, evaluated more SNPs(MMP1-1607 1G/2G, MMP2-1306 C/T, MMP2-735 C/T, MMP9 -1562 C/T and MMP13 -77A/G) and performed analysis by a comprehensive strategy, while they only analyzed single polymorphism and lung cancer risk.