In recent years, a special focus of microbiology research has been on the certain groups of bacteria (such as the superphylum Planctomycetes-Verrucomicrobia-Chlamydiae—PVC bacteria) because they exhibit certain characteristics which are unusual for prokaryotes, and which are also shared by eukaryotes. Such characteristics include a nuclear membrane, budding reproduction, sterol biosynthesis, and condensed nucleoids. These characteristics challenge the long-held concept that the presence or absence of a nuclear membrane is enough to differentiate between prokaryotes and eukaryotes and also seemingly support the evolutionary idea of the transition from prokaryotes to eukaryotes due to seemingly similar structures shared by these two domains of life. However, upon closer examination, many protein sequences which are involved in these structures and processes shared by prokaryotes and eukaryotes show low sequence homology and are similar in structure only. Therefore, these proteins can only be said to be analogous to each other, rather than homologous, which is required for evolutionary descent with modification. Furthermore, PVC bacteria are not thought to be the direct ancestors of eukaryotes, despite their analogous cellular characteristics. Also, even though the α-proteobacteria are thought to be the ancestor of the eukaryotes' mitochondria, its energetic capabilities are questionable, which are hypothesized to be necessary for the expansion of eukaryotic cell complexity, compared to those of other bacteria, such as that of species in the genus Rhodobacter. Some bacteria have also been discovered that contain energy-producing compartments called the anammoxosome, which contradicts the endosymbiotic theory, which states that energy-producing bacteria became mitochondria, which was necessary to induce eukaryotic cellular complexity.