Common fragile sites (CFSs) are expressed as chromosome gaps in cells of different species including human and mouse as a result of the inhibition of DNA replication. They may serve as hot spots for DNA breakage in processes such as tumorigenesis and chromosome evolution. Using multicolor fluorescence in situ hybridization mapping, the authors describe here human CFS FRA7K on chromosome band 7q31.1 and its murine homolog Fra12C1. Within the syntenic FRA7K/Fra12C1 region lies the IMMP2L/Immp2l gene with a size of 899/983 kb. The authors further mapped 2 amplification breakpoints of the breast cancer cell line SKBR3 to the CFSs FRA7G and FRA7H. The 5 molecularly defined CFSs on chromosome 7 do not preferentially colocalize with synteny breaks between the human and mouse genomes and with intragenomic duplications that have occurred during chromosome evolution. In addition, in contrast to all currently reported data, CFSs in chromosome band 7q31 do not show increased DNA helix flexibility in comparison with control regions without CFS expression.