Components of the Par system are conserved across metazoans. (A) The ‘Par system’ is a set of regulatory proteins that direct the polarity of the cell. For the examined bilaterian animals, the Par system is influenced by the interaction between cell-cell junctions, cytoskeletal elements, and components of the Wnt signaling pathways. (B) In bilaterian animals, Par-3, Par-6, and aPKC form a bi/tripartite complex that localizes to apical regions of the cell, binding to CDC42 and CRUMBS. In contrast, both Lgl and Par-1 localize at the basolateral cortex of the cell. Mutual antagonism by phosphorylation has been proposed as the mechanism that controls the segregation of these two distinct cortical domains. (C) Par proteins are present in the genome of sequenced Metazoa, including Cnidaria, Ctenophores, Porifera, and Placozoa. However, the function of Par proteins, in epithelial (blue branches) and early embryogenesis (red asterisk), has only been described for some bilaterian animals, and there are no descriptions available for non-bilaterian animals (black branches).