This volume is the first of a series concerning a new tech­ nology which is revolutionizing the study of biology, perhaps as profoundly as the discovery of the gene. As pointed out in the introductory chapter, we look forward to the future impact of the technology, but cannot see where it might take us. The purpose of these volumes is to follow closely the explosion of new tech­ niques and information that is occurring as a result of the newly­ acquired ability to make particular kinds of precise cuts in DNA molecules. Thus we are particularly committed to rapid publication. Jane K. Setlow Alexander Hollaender v INTRODUCTION AND HISTORICAL BACKGROUND 1 Maxine F. Singer CLONING OF DOUBLE-STRANDED cDNA. 15 Argiris Efstratiadis and Lydia Vi11a-Komaroff GENE ENRICHMENT. - -. 37 M. H. Edgell, S. Weaver, Nancy Haigwood and C. A. Hutchison III 51 TRANSFORMATION OF MAMMALIAN CELLS. M. Wig1er, A. Pe11icer, R. Axel and S. Silverstein CONSTRUCTED MUTANTS OF SIMIAN VIRUS 40 73 D. Short1e, J. Pipas, Sondra Lazarowitz, D. DiMaio and D. Nathans STRUCTURE OF CLONED GENES FROM XENOPUS: A REVIEW 93 R. H. Reeder TRANSFORMATION OF YEAST 117 Christine lIgen, P. J. Farabaugh, A. Hinnen, Jean M. Walsh and G. R. Fink THE USE OF SITE-DIRECTED MUTAGENESIS IN REVERSED GENETICS 133 C. Weissmann, S. Nagata, T. Taniguchi, H. Weber and F. Meyer AGROBACTERIUM TUMOR INDUCING PLASMIDS: POTENTIAL VECTORS FOR THE GENETIC ENGINEERING OF PLANTS. 151 P. J. J. Hooykaas, R. A. Schi1peroort and A.