Advances in the in vitro synthesis and evolution of DNA, RNA, and polypeptides are accelerating the construction of biopolymers, pathways, and organisms with novel functions. Known functions are being integrated and debugged with the aim of synthesizing life-like systems. The goals are knowledge, tools, smart materials, and therapies. Synthetic biology projects (SBPs) The basic elements of chemistry and biology are few, but the synthetic combinations are unlimited and awe inspiring. The first

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... ternational conference on synthetic biology charted its goals as understanding and utilizing life's diverse solutions to process information, materials, and energy (Silver and Way 2004) (http://syntheticbiology.org). As a bonus, genetic systems are biocompatible, renewable, and can be optimized by Darwinian selections. SBPs entail the complex manipulation of replicating systems, ranging from the sophisticated genetic engineering of organisms (Zimmer 2003; Ferber 2004; Gibbs 2004) to the chemical synthesis of unnatural replication (Rawls 2000; Szostak et al. 2001; Benner and Sismour 2005) . It thus seems appropriate to divide synthetic biology into two classes, in vivo and in vitro. In vitro SBPs have received considerably less attention, so we focus on them here after introducing in vivo SBPs to illustrate the differences between the two classes. 3 Corresponding authors. E-mail a.forster@vanderbilt.edu; fax (615) 936-5555. E-mail: http://arep.med.harvard.edu/; fax (617) 432-6513. Article published online before print. Article and publication date are at