Cochliobolus carbonum is a fungus that causes northern leaf spot and ear rot disease in corn. The fungus produces a toxin that is highly destructive to corn ears and leaves. The HM1 gene in corn is responsible for resistance or susceptibility to the fungal plant pathogen. Corn lines that are resistant to C. carbonum and its toxin can become susceptible if their HM1 gene is mutated. This seemingly unfortunate event has allowed maize researchers to clone and characterize the HM1 gene, which led to a better understanding of how fungal toxins work and how plants defend themselves against pathogens. Mutant analysis is one of the most powerful tools researchers have for understanding how genes work and how their expression controls different pathways and how plants respond to the environment.