GSTM1 and laryngeal carcinoma: Of the 24 articles selected, 4 studies were excluded because of the lack of data about GSTM1 status of laryngeal cancer [4], [17], [18], [19], 2 studies [20], [21] were excluded by reason of their data overlapped with that of other two studies respectively [22], [23], study by Hanna et al. [24]was excluded because polymorphism was determined in fresh frozen tissue specimens.