The X-linked form of Emery-Dreifuss muscular dystrophy (EDMD) was recently shown to be due to mutations in the STA gene on chromosome Xq28. We have demonstrated a simple test for the diagnosis of this condition, looking for altered expression of the protein, emerin, in leucocytes and skin with a monoclonal antibody. Full-length emerin is completely absent in affected boys from the EDMD families studied. The method has also enabled identification of a female carrier of the disease by reduced levels of the protein on the leucocyte Western blot and a mosaic pattern of expression by immunofluorescence microscopy of the skin biopsy.