Expression of common fragile sites on the X chromosome corresponds with active gene regions

The expression of common chromosomal fragile sites on human chromosomes has been proposed to be a cytogenetic expression of gene activity. Distinctive patterns of expression of two common fragile sites on the human X chromosome were observed in females. The fragile site at Xp22.31, located in a band region that contains genes which escape X inactivation, was expressed on both X chromosomes. By contrast, the fragile site at Xq22.1, in a region assumed to be subject to X inactivation, was expressed almost exclusively on one X, the active X chromosome. These findings provide evidence that common fragile site expression only occurs in regions with active genes.