In some neurodevelopmental conditions such as paediatric leukoencephalopathies, homozygous mutations in the colony-stimulating factor 1 receptor (CSF1R) gene lead to a permanent absence of microglia which corresponds to overt structural abnormalities in the corpus callosum, the cerebellum and the ventricles. The gene discussed is CSF1R; the disease is Leukoencephalopathy.