GSTM1 and chronic obstructive pulmonary disease: So far, previous linkage analysis studies have identified many candidate genes that are associated with the development of COPD, such as PI MZ α-1 antitrypsin gene [38], matrix metalloproteinase 12 (MMP12) [39], transforming growth factor β-1 (TGFB1) and glutathione S-transferase Mu 1 (GSTM1) [40].