A reconstituted human breast tumor model is disclosed. The model, which is incorporated into mice, provides actual tumors that arise spontaneously, thereby mimicking naturally occurring breast cancer. The tumors are genetically human, because they arise from human mammary tissues that develop from human mammary epithelial cells implanted into host mice. Prior to implantation, the mammary epithelial cells are genetically modified to contain a recombinant human oncogene and an SV40 early region.