“…Common fragile sites are considered part of the chromosome structure since they have been described in different mammalian species (Rodentia [10], Carnivora [11,12], Perissodactyla [13], Cetartiodactyla [14] and Primates [7,15,16]), whereas rare fragile sites are found expressed in a small percentage of the human population [17]. In total, 21 human fragile sites have been molecularly characterized: eight rare fragile sites (FRAXA [18], FRAXE [19], FRAXF [20], FRA10A [21], FRA10B [22], FRA11B [23], FRA16B [24], and FRA16A [25]), and 13 common human fragile sites (FRA1E [26], FRA2G [27], FRA3B [28], FRA4F [29], FRA6E [30], FRA6F [31], FRA7E [32], FRA7G [33], FRA7H [34], FRA9E [35], FRA13A [36], FRA16D [37], and FRAXB [38]). Whereas the expression of rare fragile sites is known to be related to the amplification of specific repeat motifs (CCG repeats and AT-rich regions), no simple repeat sequences have been found to be responsible for the instability observed at common fragile sites.…”