One piece of genetic evidence for the biological distinctness of
schizophrenia and bipolar illness is the rarity of monozygotic twin pairs
in which one twin suffers from schizophrenia and the other from bipolar
disorder. The authors describe a pair of monozygotic mirror- image twins
with discordant diagnoses, schizophrenia in one twin and bipolar or
schizoaffective disorder in the other.