Chromosomes of cells grown in culture sometimes develop constrictions or gaps at particular locations, called fragile sites (Figure 6.17) because they are prone to breakage under certain conditions. More than 100 fragile sites have been identified on human chromosomes. One of the most intensively studied is a fragile site on the human X chromosome that is associated with fragile-X syndrome, a disorder that includes intellectual disability. Fragile-X syndrome, which exhibits X-linked inheritance and arises with a frequency of about 1 in 5000 male births, has been shown to result from an increase in the number of repeats of a CGG trinucleotide (see Chapter 13). Other common fragile sites do not consist of trinucleotide repeats, however, and their nature is still not completely understood.