Mammary epithelium can be reconstituted in vivo by transplanting fragments of mammary epithelium, or suspensions of mammary epithelial cells, into the 'cleared' mammary fat pad of a syngeneic recipient mouse. A 'cleared' mammary fat pad is one from which the natural epithelium has been removed at 3 weeks of age. Genes can be introduced into the epithelium before transplantation using retrovirus vectors, or the epithelium can be taken from a knockout mouse 1. The applications of transplantation, and its advantages and disadvantages compared with transgenesis will be surveyed, including the ability to use hormone-insensitive promoters; to introduce genes into clones of cells rather than whole tissues; the ease of studying early preneoplastic change; and the use of transplantation with transgenic knockouts, to rescue embryonic lethals and to distinguish systemic from local effects 2.