Cell transplantation provides a way to study genes that may be important in human tissue aging. Studies on gene action in human cells are usually restricted to cell culture investigations and clinical observations. Differences in human and rodent cellular biology, particularly with respect to telomere dynamics, show the need for new systems for investigating aging that use human cells or cells of other large, long-lived mammals, such as bovine cells. The system we describe uses human and bovine adrenocortical cells transplanted into scid (severe combined immunodeficiency) mice. They form a vascularized tissue structure that can replace the essential functions of the animals' own adrenal glands. The cells may be genetically modified before introduction into the animal. Using hTERT (telomerase reverse transcriptase) and oncoproteins, we show the potential for investigating gene action in genetically modified tissues created by cell transplantation.