FOXG1 and Down syndrome: However, due to the significant differences in neurological phenotypes of the underlying FOXG1 mutations (e.g., agenesis of the corpus callosum, blunted gyrification, and reduction in white matter volume in some cases) compared to MECP2-mediated Rett syndrome, “FOXG1 syndrome” is now considered a distinct disorder (Kortüm et al., 2011).