1) Specific loci that show up during karyotyping as a gap (an uncondensed stretch in closer views) on a chromatid arm after culturing cells under specific conditions. These sites are associated with an increase in chromosome fragility. They are classified as common or rare, and by the specific culture conditions under which they develop. Fragile site loci are named by the letters "FRA" followed by a designation for the specific chromosome, and a letter which refers to which fragile site of that chromosome (e.g. FRAXA refers to fragile site A on the X chromosome. It is a rare, folic acid-sensitive fragile site associated with fragile X syndrome. )

2) Heritable sensitive regions of chromosomes which show up in vitro as non-staining bands. They are associated with chromosome breakage and other aberrations. No abnormal phenotype has been definitely identified with autosomal fragile sites, but some rare autosomal recessive disorders may be due to homozygosity for fragile sites. A fragile site on the X chromosome is associated with Fragile X Syndrome. Fragile sites are designated by the letters "fra" followed by the designation for the specific chromosome and locus.