Fragile X chromosome: An X chromosome with a heritable fragile
site associated with a frequent form of mental retardation -- the fragile X syndrome.
The fragile X chromosome was first sighted by Dr. Herbert A. Lubs in 1969 who termed it the "marker X.". The fragile X is also called FRAXA. It was the first fragile site found on the X chromosome.
The fragile X chromosome is due to a trinucleotide repeat sequence (a recurring motif of 3 bases) in the DNA at that spot.