ELISA using the immunodominant genus-specific, 56-kDa, type-specific antigen (outer membrane protein [Omp]) does not react with antibodies produced during typhus group rickettsiae (TGR) and spotted fever group rickettsiae (SFGR) infections because rickettsiae lack the 56-kDa type-specific antigen. The gene discussed is OMP; the disease is typhus.