Update on designing and building minimal cells.

Minimal cells comprise only the genes and biomolecular machinery necessary for basic life. Synthesizing minimal and minimized cells will improve understanding of core biology, enhance development of biotechnology strains of bacteria, and enable evolutionary optimization of natural and unnatural biopolymers. Design and construction of minimal cells is proceeding in two different directions: 'top-down' reduction of bacterial genomes in vivo and 'bottom-up' integration of DNA/RNA/protein/membrane syntheses in vitro. Major progress in the past 5 years has occurred in synthetic genomics, minimization of the Escherichia coli genome, sequencing of minimal bacterial endosymbionts, identification of essential genes, and integration of biochemical systems.