Technical Abstract: The hypersensitive response (HR) is the most visible and arguably the most important defense response in plants, although the details of how it is controlled and executed remain patchy. In this paper a novel genetic technique called MAGIC (Mutant-Assisted Gene Identification and Characterization) is used to identify an HR-modulating locus in maize. MAGIC facilitates the identification of naturally occurring useful alleles from diverse germplasm using a mutant phenotype as a “reporter”. In this case the reporter phenotype is caused by a partially-dominant autoactive disease resistance-gene Rp1-D21 which causes HR lesions to form spontaneously all over the plant in a developmentally specified manner. We demonstrate that the Rp1-D21 phenotype is profoundly affected by genetic background. This includes the genomes of B73 and Mo17, two popular maize inbreds that partially suppress and enhance the Rp1-D21 phenotype, respectively. By crossing the Rp1-D21 gene into the IBM mapping population it was possible to map and identify Hrml1 on chromosome 10, a locus responsible for modulating the Rp1-D21 phenotype. Other loci with smaller effects were identified on chromosomes 1 and 9. These results demonstrate that MAGIC is a viable approach for identifying additional genetic components underlying HR.