In the study of biological systems, often the amount of starting material available for molecular analysis is limiting. In some situations, only a few cells in a tissue are expressing genes of interest or the tissue is in limited supply. A cDNA library from the targeted cells is preferable to a library constructed from the entire tissue containing these cells, because in the latter case, genes expressed in the targeted cells may be too rare to be detected. cDNA clones of genes expressed in small amounts of material are often hard to obtain because the construction of conventional cDNA libraries requires microgram amounts of poly A⁺ RNA (1 ).