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.