Although more than 100 mutations have been identified in the copper/zinc superoxide dismutase (Cu/Zn-SOD) in familial amyotrophic lateral sclerosis (FALS), the mechanism responsible for FALS remains unclear. The finding of the present study shows that FALS-causing mutant Cu/Zn-SOD proteins (FALS mutant SODs), but not wild-type SOD, are barely detected by three monoclonal antibodies (mAbs) in Western blot analyses. The enzyme-linked immunosorbent assay for denatured FALS mutant SODs by dithiothreitol, SDS, or heat treatment also showed a lowered immunoreactivity against the mAbs compared with wild-type SOD. Because all the epitopes of these mAbs are mapped within the Greek key loop (residues 102-115 in human Cu/Zn-SOD), these data suggest that different conformational changes occur in the loop between wild-type and FALS mutant SODs during the unfolding process. Circular dichroism measurements revealed that the FALS mutant SODs are sensitive to denaturation by dithiothreitol, SDS, or heat treatment, but these results do not completely explain the different recognition by the mAbs between wild-type and FALS mutant SODs under the denatured conditions. The study on the conformational changes in local areas monitoring with mAbs may provide a new insight into the etiology of FALS.