Phylogenetic analysis of the endo-beta-1,4-glucanase gene family of Arabidopsis and other plants revealed a clear distinction in three subfamilies (alpha, beta, and gamma). The alpha- and beta-subfamily contains proteins believed to be involved in a number of physiological roles such as elongation, ripening, and abscission. The gamma-subfamily is composed of proteins that are predicted to have a membrane-spanning domain and to be localized at the plasma membrane. Some of these proteins have been linked to cellulose biosynthesis by serving to hydrolyze a lipid-linked intermediate that acts as a primer for the elongation of beta-glucan chains during cellulose synthesis at the plasma membrane. Similar glucanases are important in cellulose biosynthesis in bacteria. Searches in the genomes of unrelated organisms that make cellulose, such as Ciona intestinalis and Dictyostelium discoideum, revealed the presence of membrane-linked endo-beta-1,4-glucanases and it is suggested that these might also have a role in cellulose synthesis.