A prokaryote, less commonly spelled procaryote, is a cellular organism that lacks a distinct cell nucleus or other membrane-bound organelles. In the earlier two-empire system, prokaryotes formed the empire Prokaryota; in the current three-domain system, based upon molecular phylogenetics, prokaryotes are divided into two domains Bacteria and Archaea. A third domain, Eukaryota, which consists of single- and multi-cellular organisms with nuclei, organelles and complex cellular architecture, is sometimes considered a clade under Archaea in the two-domain system.