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Fragile X syndrome
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Fragile X syndrome, formerly called Martin-Bell syndrome, is a triplet repeat expansion disorder: a three-letter sequence of DNA that repeats more times than it should. It is the most common inherited cause of intellectual disability, from mild to severe.
The triplet in question is the letters CGG, inside the FMR1 gene. The direct cause of the syndrome is the complete or near-complete absence of that gene's FMRP protein. And this is no ordinary protein: it is a master regulator that directs translation, the step in which the cell builds its proteins. It is like a syllable repeated one too many times until the sentence can no longer be read.
The count matters. People without the disease carry between 5 and 44 repeats, and more than 200 repeats marks the full mutation.
In brief
Fragile X syndrome, a triplet repeat expansion disorder, is the most common inherited cause of mild to severe intellectual disability.
It is caused directly by the complete or near-complete absence of FMRP, the protein encoded by FMR1 that directs protein translation.
A syllable repeated one too many times until the sentence can no longer be read.
The repeat count defines the diagnosis: unaffected people carry 5 to 44 repeats, and the full mutation exceeds 200.
Sources (1)
- National Center for Biotechnology Information, StatPearls PublishingFragile X Syndrome, StatPearlsPublished
This page explains the topic in general terms; it does not tell anyone what treatment to follow.
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