Endogenous plasmids are found in most strains of Rhodobacter sphaeroides, Rhodobacter capsulatus and Rhodospirillum rubrum, but appear to be less common in other species of purple non-sulphur bacteria. They have also been found in some species of green and purple sulphur bacteria. Until recently, there was little conclusive evidence for the functions of these plasmids, and most were considered to be cryptic. The advent of whole genome sequencing has enabled predictions of possible plasmid functions that can then be tested experimentally. In addition, transcriptomic and proteomic studies in R. sphaeroides 2.4.1 have shown that plasmids play an active role in genome function and may code for essential metabolic processes.