The acrocallosal syndrome is an autosomal recessive mental retardation syndrome with brain abnormalities such as corpus callosum agenesis and/or Dandy-Walker malformation as well as dysmorphic features, postaxial polydactyly of the hands, and preaxial polydactyly of the feet. It is considered a ciliopathy. Joubert syndrome-12 is a disorder with an overlapping phenotype characterized by the hallmark finding of the molar tooth sign (MTS) on brain MRI. [From OMIM]