 Fibrillarin is associated with U3, U8 and U13 small nuclear RNAs in mammals and is similar to the yeast NOP1 protein.Fibrillarin has a well conserved sequence of around 320 amino acids, and contains 3 domains, an N-terminal Gly/Arg-rich region; a central domain resembling other RNA-binding proteins and containing an RNP-2-like consensus sequence; and a C-terminal alpha-helical domain.An evolutionarily related pre-rRNA processing protein, which lacks the Gly/Arg-rich domain, has been found in various archaea.