GSTM1 and asthma: In a study by Piirilä et al. 182 workers exposed to different isocyanates (HDI, TDI and MDI) were screened for polymorphisms in the four glutathione S-transferase (GST) supergene family genes GSTM1, GSTM3, GSTP1 and GSTT1, and 109 workers showed signs of isocyanate induced asthma, while 73 did not.