ABSTRACT.Two mentally handicapped dizygotic twin sisters were found to possess the Fragile‐X lesion. They showed different levels of cognitive deficit. The hypothesis that the level of cognitive function in female carriers of the Fragile‐X lesion may be influenced by Lyonization is discussed. The mother of the twins was schizophrenic. Although she was an obligate carrier her psychosis was thought not to be causally connected with the Fragile‐X s