Melanotic tumour formation and lethality are due to the same mutation. Mutations exhibit incomplete penetrance and expressivity of the melanotic tumour phenotype. Small and/or abnormally shaped brain. Portion of alimentary canal posterior to the junction between the mid and hind gut is melanized. Hemizygous males exhibit nonhypertrophied lymph glands with melanized nodules. Lymph glands had an increased number of lobes.