Western blotting is utilized to detect and quantify the amount of a specific protein in a complex mixture of proteins and at the same time to determine its molecular weight. In particular, by Western blotting it is also possible to detect and discriminate the two isoforms of nm23, H1, and H2 (1, 2). nm23-H1 is about 18.5 kD (3 ) and nm23-H2 is 17 kD (4 ), hence, they migrate as two distinct bands on a standard SDS polyacrylamide gel. Because the two isoforms are very highly conserved (4 ), they retain most of the same epitopes; thus any polyclonal antibody raised against one of the two isoforms or a monoclonal antibody against a conserved epitope will recognize both nm23-H1 and -H2 at the same time. Analysis by immunohistochemistry, on the other hand would require two distinct reactions with two specific antibodies for H1 and H2 raised against the least conserved epitopes to avoid crossreaction.