In a study of Caucasians, the GSTM1 null genotype was associated with a 1.89-fold higher risk of diisocyanate-induced asthma (n = 109) compared with TDI exposure in the absence of asthma (n = 73), while the frequency of the GSTT1 null genotype did not differ between the two groups [27].These findings are in line with our finding that the GSTM1 null genotype was associated with an approximately three-fold increased risk of TDI-induced asthma, while the GSTT1 null genotype had an insignificant association. This evidence concerns the gene GSTM1 and asthma.