FRAGILE X CHROMOSOME

X chromosome with a fragile site associated with a frequent form of mental retardation. The fragile X chromosome was first sighted by Herbert A. Lubs in 1969. The fragile X is also called FRAXA (the second A signifies it was the first FRAgile site found on the X chromosome). It is due a trinucleotide repeat (a recurring motif of 3 bases) in the DNA at that spot.