Dizygosity of discordant twins with Noonan syndrome.

A pair of discordant twins, one of whom had Noonan syndrome, is reported. Most of the cardinal signs of Noonan syndrome were demonstrated by the affected twin. The etiology of the syndrome is briefly reviewed. The dizygosity of the twins was proved by ABO blood grouping and mixed lymphocyte reaction. The findings are interpreted as reasonably good evidence that genetic factors are the prime etiology in the pathogenesis of Noonan syndrome.