Normal early development has generally been insisted on as an essential criterion for the diagnosis of Rett syndrome. A new set of monozygotic female twins is reported. Twin 1 was considered to be abnormal from birth while delay was not suspected in twin 2 until she was about one year old. Some regression occurred during the second year in both twins, who are now clinically indistinguishable from each other at age 4 years. Other than a slight difference in head circumference at birth, no environmental factor which could account for the clinical difference has been identified.

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