In recent years, we have witnessed an explosion in the number of techniques available for selectively perturbing the genes and molecules that regulate tissue growth and maintenance. From drugs and neutralizing antibodies, we have progressed to gene deletion, “dominant negative,” antisense, and ribosymal strategies. As advances are made in chromosomal mapping and analysis, we are progressively more capable of creating accurate animal models of disease by means such as the site-directed mutagenesis of DNA and the creation of transgenic animals. Allied to this have been rapid developments in the means by which additional genes can be engineered into cells in tissue culture prior to their transplantation or integration into living animals. This latter technique often includes an inducible means of identifying the foreign cells in tissues by such methods as staining for β-galactosidase. In these studies, a commonly asked question is, “What has happened to the foreign cells?” Have they increased or decreased in number? Have they induced changes in the numbers of cells surrounding, or been influenced by them?