Corpus callosum agenesis is a birth defect in which the structure that connects the two sides of the brain (the corpus callosum) is partially or completely absent. This birth defect can occur as an isolated condition or combined with other cerebral abnormalities, including Arnold-Chiari malformation, Dandy-Walker syndrome, schizencephaly (clefts or deep divisions in brain tissue), and holoprosencephaly (failure of the forebrain to divide into lobes.) It may also occur as part of other diseases such as Aicardi syndrome, (which only affect girls, includes Corpus callosum agenesis, and other problems) or Andermann syndrome or it can also be associated with malformations in other parts of the body, such as midline facial defects. Symptoms are vary from person to person. Many people with agenesis of the corpus callosum do not have any symptoms or the symptoms may range from subtle or mild to severe, depending on whether and which associated brain abnormalities are present. The exact cause is unknown.