By discovering a mutant mouse that is highly susceptible to mammary tumors, Cornell researchers have found a novel potential link between genetic defects in DNA replication (copying) and breast cancer. The mouse contains a mutation in a gene essential for replicating DNA in both humans and mice that increases the rate of flawed copies of genetic material, leading to mouse breast cancers.
http://news.cornell.edu/stories/2006/12/mutant-mouse-provides-insights-breast-cancer