The UI-R-C4 library is a subtracted library of a series, ultimately derived from a mixture of tissues from rat placenta, adult lung, brain, liver, kidney, heart, spleen, ovary, muscle, and 8, 12 and 18-day embryos. For a detailed description of the library from which this clone was derived, please visit our web site at ratest.eng.uiowa.edu. This procedure has been previously described (Bonaldo, Lennon and Soares, Genome Research 6:791-806, 1996)