1. Correlations between deletions in two glutathione S-transferase (GST) genes, GSTM1 and GSTT1 and susceptibility to Alzheimer's disease (AD), motor neuron disease (MND) and Parkinson's disease (PD) have been investigated by PCR, using primers specific for both genes. 2. It was found that males with a deletion of the GSTM1 gene were more susceptible to PD and males with a deletion of the GSTT1 gene more susceptible to MND and PD, possibly implying that environmental factors which specifically target men may be involved. Furthermore, subjects with a deletion of the GSTT1 gene were more susceptible to AD.