Animal cells are cultured in vitro for a variety of uses. Advances in recombinant DNA technologies have resulted in an array of heterologous proteins being produced from cultured cells for use as research tools, potential biotherapeutics (1), or analytical reagents. Cells are also used as products in their own right in receptor research, to study xenobiotic metabolism and toxicity in vitro (2), in high throughput screening, and for biopharmaceutical biological activity assays.