KRAS and breast cancer: There are two additional reasons to choose this oncogene: (i) Viral vector-mediated delivery of a mutated member of the Ras family (Kras, Hras, or Nras) has been reported to cause mammary tumors in mice and rats swiftly (1, 19, 24); (ii) we wanted to compare our result with the previous Platt et al. (15) report on somatic editing of Kras to generate lung tumors.