Silencing of the FMR1 gene in<unk>  X syndrome. FMR1 co-localizes with a rare<unk> <unk> , visible here as a gap on the long arms of the X chromosome. A<unk>  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,<unk> <unk> s are classified as "common" or "rare". To date, more than 120<unk> <unk> s have been identified in the human genome. Common<unk> <unk> s are considered part of normal chromosome structure and are present in all (or nearly all) individuals in a population. Under normal conditions, most common<unk> <unk> s are not prone to spontaneous breaks. Common<unk> <unk> s are of interest in cancer studies because they are frequently affected in cancer and they can be found in healthy individuals.<unk> s FRA3B (harboring the FHIT gene) and FRA16D (harboring the WWOX gene) are two well known examples and have been a major focus of research. Rare<unk> <unk> s are found in less than 5% of the population, and are often composed of two- or three-nucleotide repeats. They are often susceptible to spontaneous breakage during replication, frequently affecting neighboring genes. Clinically, the most important rare<unk> <unk>  is FRAXA in the FMR1 gene, which is associated with the<unk>  X syndrome, the most common cause of hereditary intellectual disability. For detailed information regarding each characterized<unk> <unk> , please visit HumCFS: a database of<unk> <unk> s in human chromosomes, published in BMC Genomics.
Chromosomal fragile site
Germline mutation
Chromosomal translocation
Dichromacy
Genetic predisposition
Germline mosaicism
Small supernumerary marker chromosome
Chromosome abnormality