It has been known for some time that steroid and thyroid hormones can regulate cell proliferation (1 ,2 ) by stimulating cell division, as in the case of the thyroid hormone; by preventing proliferation, as in the case of the glucocorticoid hormone; or by inducing differentiation, as in the case of retinoic acid. Recent studies of nuclear hormone receptors have shown that hormone function is mediated by nuclear hormone receptors acting as transcriptional enhancers to stimulate expression of a set of genes (1 ). The genes for nuclear hormone receptors and for receptors of other regulatory molecules appear to have a common underlying structure (1 ,2 ), which in turn suggests that these genes have evolved as duplications of a single ancestral gene. Recent studies of the complex interactions of these receptors with each other and with their DNA targets have led to some understanding of these interactions and their role in tumorigenesis and tumor progression (3 ).