Human colorectal tumor xenografts in nude mice: expression of malignancy.

Factors associated with the growth and progression of (human) tumor xenografts have been briefly reviewed. Intrinsic properties of tumor cells, most of them yet to be recognized, may interfere with multiple rejection processes and/or growth regulatory signals developed in various immunodeficient recipients. Using alternative hosts or new experimental conditions, it is hoped that the system can provide valuable information on the tumor invasive and metastasizing processes.