The genome of Mycoplasma genitalium is so far the smallest discovered for any free-living organism, so it has been used as a starting point for defining a minimal genome. Transposon mutagenesis and comparison with a second mycoplasma have further narrowed down the list of genes. Now Glass et al. announce the sequencing of a third mycoplasma, the mucosal pathogen Ureaplasma urealyticum, in the 12 October Nature (Nature 2000, 407:757-762). Their results suggest that there is more than one version of a minimal genome, even for organisms living in very similar environments. Although U. urealyticum has homologs for most of the M. genitalium genes that have been proposed to be essential, it lacks the heat shock protein/chaperonins GroEL and GroES (found in all other sequenced microbial genomes) and the cell division protein FtsZ (absent only in a single archaeon and in chlamydia, which divide in host vacuoles)....