Article Text

Download PDFPDF
Twin sisters, monozygotic with the fragile X mutation, but with a different phenotype
  1. Rob Willemsen,
  2. Renske Olmer,
  3. Yolanda De Diego Otero,
  4. Ben A Oostra
  1. CBG Department of Clinical Genetics, Erasmus University, PO Box 1738, 3000 DR Rotterdam, The Netherlands
  1. Dr Oostra, oostra{at}


The absence of the fragile X mental retardation protein (FMRP) results in fragile X syndrome. All males with a full mutation in theFMR1 gene and an inactiveFMR1 gene are mentally retarded while 60% of the females with a full mutation are affected. Here we describe monozygotic twin sisters who both have a full mutation in theirFMR1 gene, one of whom is normal while the other is affected. Using molecular and protein studies it was shown that owing to preferential X inactivation in the affected female a minority of the cells expressed the normalFMR1 gene, while in her sister most cells expressed the normal FMR1 gene. This shows that X inactivation took place in the female twins after separation of the embryos and that for a normal phenotypeFMR1 expression is necessary in the majority of cells.

  • fragile X syndrome
  • mental retardation
  • monozygotic twins
  • Lyonisation

Statistics from

Request Permissions

If you wish to reuse any or all of this article please use the link below which will take you to the Copyright Clearance Center’s RightsLink service. You will be able to get a quick price and instant permission to reuse the content in many different ways.