The identification of specific messenger RNA molecules and the characterization of the proteins encoded by them has been greatly assisted by the development of in vitro translation systems. These cell-free extracts comprise the cellular components necessary for protein synthesis, i.e., ribosomes, tRNA, rRNA, amino acids, initiation, elongation and termination factors, and the energy-generating system (1 ). Heterologous mRNAs are faithfully and efficiently translated in extracts of HeLa cells (2 ), Krebs II ascites tumor cells (2 ), mouse L cells (2 ), rat and mouse liver cells (3 ), Chinese hamster ovary (CHO) cells (2 ), and rabbit reticulocyte lysates (2 ,4 ), in addition to those of rye embryo (5 ) and wheat germ (6 ). Translation in cell-free systems is simpler and more rapid (60 min vs 24 h) than the in vivo translation system using Xenopus oocytes.