Fragile sites update

Abstract

Fragile sites are important as chromosome markers. At least 104 fragile sites are now known. These comprise 24 rare and 80 common fragile sites. The locations of all 104 fragile sites are given here, together with their frequencies and types as to mode of induction. There is apparent nonrandom distribution of fragile sites in the genome. The agent 5-azacytidine seems preferentially to induce expression of fragile sites in variable heterochromatic regions. Fragile sites from diverse classes tend to be in coincident locations, suggesting double ascertainment of certain fragile sites and the existence of multiple alleles of fragile sites at some loci.