Engineering a cell with the minimum number of genes, functions or components required for living in a defined environment has become one of the promises of system biology. Theoretical as well as experimental approaches have been undertaken in order to identify essential genes and functions for different model organisms. These experiments have shown that, despite the well-known group of universally conserved genes mainly related to transcription and translation, there is a fraction of essential

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... enes which are particular to each one of the species analyzed, thus defining a diversity of possible minimal genomes. Comparative analysis of naturally evolved near-minimal genomes have the potential to uncover how nature has solved in different ways the accomplishment of basic functions to sustain a cell. Here we review the state of the art of reduced sequenced genomes and discuss the concept of a minimal cell at the light of this biodiversity.