Common fragile sites are considered part of normal chromosome structure and are present in all (or nearly all) individuals in a population. Under normal conditions, most common fragile sites are not prone to spontaneous breaks. Common fragile sites are of interest in cancer studies because they are frequently affected in cancer and they can be found in healthy individuals. Sites FRA3B (harboring the "FHIT" gene) and FRA16D (harboring the "WWOX" gene) are two well known examples and have been a major focus of research.