"In higher organisms, the cells of an individual become greatly diversified despite their identity of . How such diversification is achieved and how supra-cellular organization then comes about have remained largely obscure. A new way of getting at theses questions was formulated in ; its purpose was to subject the pivotal genotype-phenotype relationship to experimental manipulation. The intact organism was taken to be the necessary framework for such an experimental study of gene expression, and the mouse, with its wide variety of available s, was easily the most promising vertebrate species. The plan was to make artificial mice: within each, cells with different, rather than identical, genotypes would be included. ... Certain kinds of had previously been extensively employed in studies with '. ... The first indication, in a mammal, of an admixture of genotypes came with 's discovery of erythrocyte mosaicism in fraternal cattle co-twins. ..."