Specific antibody response against an alien protein is one of the basic immunologic mechanisms in immunecompetent organisms. They can be used as a first step in various approaches leading to the identification of proteins or even an antigen-encoding gene. Accordingly, we wanted to find out whether a null-mutant immunecompetent organism would produce specific antibodies against the missing gene product. We chose the mouse mutant mdx (X-linked muscular dystrophy) which represents a null-mutant condition for the gene product of the Duchenne muscular dystrophy (DMD) gene, dystrophin. When dystrophin-deficient mdx mice received dystrophin-containing muscle grafts from coisogenic normal mice, high titres of antibodies specific for dystrophin were detected in the transplanted animals' sera. Because dystrophin-containing muscle grafts were not rejected but have properly regenerated even in the presence of high titre antibodies against dystrophin, these findings have important bearings on all therapeutical strategies based on dystrophin supplementation. Using the mdx mouse as null-mutant model we showed that there was no immune tolerance for the missing protein but specific antibodies were produced when the organism came in contact with this protein. This simple approach may serve as a shortcut for identifying missing proteins presumably not only in neuromuscular disorders but in a wide range of diseases where null-mutant animal models and corresponding coisogenic inbred strains exist.