Fibrillarin / Nop1p, was originally identified as a nucleolar protein of baker's yeast, Saccharomyces cerevisiae (accession P15646). The Nop1p protein is essential for yeast viability and is localized in the nucleoli. The human homologue of Nop1p is Fibrillarin (accession P22087), a component of the nucleolar small nuclear ribonucleoprotein (snRNP) particle. The human Fibrillarin gene is located on chromosome 19 (19q13.1). Fibrillarin proteins have been cloned and sequenced from several other species (mouse, accession P35550, Xenopus accession P22232, C. elegans accession Q22053, and S. pombe accession P35551). The N terminal 80 amino acids contain multiple copies based on the peptide RGG, and the remaining 240 amino acids consist of the Fibrillarin domain. A Fibrillarin homologue has also been identified in the genome of the archean Methanococcus (accession NC_000909). This protein lacks the RGG rich N-terminal extension but is clearly homologous to the other sequences throughout the entire Fibrillarin domain. The structure of this molecule has been determined and shown to consist of 2 extended beta-sheets flanked by 4 alpha-helixes. Patients with the autoimmune disease scleroderma often have strong circulating autoantibodies to a 34kDa protein which was subsequently found to be Fibrillarin. Fibrillarin is an excellent marker for the nucleolus.