Mutations in DJ-1 can cause early onset parkinsonism. Various antibodies have been generated to detect this protein, one of which is a commonly used monoclonal antibody (clone 3E8). Since results of in situ examinations of DJ-1 expression with this antibody have differed from analyses with species-specific antibodies (e.g. rat), it would be useful to know the epitope for this antibody. Using GFP-tagged deletion constructs of human DJ-1, we have localized the epitope region for this antibody to within residues 56-78 of human DJ-1. Mapping this region to the published three-dimensional structure of DJ-1 indicates that this is a solvent-accessible surface epitope. Immunonegativity of E64D mutant DJ-1 with the monoclonal antibody suggests that glutamate 64 of human DJ-1 contributes to the epitope recognized by this antibody. Moreover, the loss of immunoreactivity due to such a small substitution demonstrates the remarkable sensitivity of the monoclonal antibody 3E8 to DJ-1.