The Kingdom Monera, as defined in the five-kingdom classification system, includes all prokaryotic organisms - primarily bacteria and cyanobacteria (formerly called blue-green algae). These organisms lack a membrane-bound nucleus and other membrane-bound organelles. Viruses are not included in any kingdom as they are non-living, protists are eukaryotic organisms in a separate kingdom, and yeast are fungi. Therefore, only bacteria correctly belong to Monera.