We have reported the use of vectors based on avian leukosis virus (ALV) and ALV env pseudotyped lentivirus for stably introducing genes into somatic mammary cells in vivo for tumor modeling. These vectors require tva transgenic mice for viral infection. Here, we report, and described the method for, stably introducing genes into the mammary gland in mice using intraductal injection of a common lentiviral vector. This method does not need tva-transgenic mice for infection. This in vivo transformation method may be useful for rapid testing of the tumorigenic potential of genes potentially involved in breast carcinogenesis.