Chromosomal fragile site
![Silencing of the FMR1 gene in Fragile X syndrome. FMR1 co-localizes with a rare fragile site, visible here as a gap on the long arms of the X chromosome.](/Images/godic/202501/14/FragileX5730.png")
A chromosomal fragile site is a specific heritable point on a chromosome that tends to form a gap or constriction and may tend to break when the cell is exposed to partial replication stress. Based on their frequency, fragile sites are classified as "common" or "rare". To date, more than 120 fragile sites have been identified in the human genome.