Fragile sites, cancer chromosome breakpoints, and oncogenes all cluster in light G bands.

Fragile sites tend to be bands where breaks occur in cancer chromosome rearrangements that can involve oncogenes. The locations of fragile sites, cancer breakpoints, and oncogenes were therefore charted. All were predominantly in light G bands. Specifically, 78 of 89 (88%) fragile sites were in light G bands including the large group of common fragile sites inducible with aphidicolin (p less than 0.001). Of 61 cancer breakpoints, 50 (82%) were in light bands including translocation breakpoints (p less than 0.001). Thirteen of 14 (93%) oncogenes localized to light bands. The sharing of chromosome bands can stem from a biologically meaningful relationship, as between cancer breakpoints and oncogenes. Joint occupancy of chromosome bands can also reflect independent reasons to be in the same sector of the genome. Thus, fragile sites may well be in light bands because they are associated with active genes. This clearly does not rule out a biologic relationship between specific fragile sites and specific cancer breakpoints.