Definition: Specific locations on a chromosome that are more likely to break during cell replication. Some fragile sites are very common in the general population, whereas others are rare and associated with specific conditions such as fragile X syndrome. These rare fragile sites are often due to sequences of three chemical bases (trinucleotide repeats) that are repeated more times than is typical. ning gap of variable width that usually involves both chromatids and is always at exactly the same point on a specific chromosome derived from an individual or kindred.