Fibrillarin is a widely occurring, basic, nonhistone protein that is localized exclusively in the fibrillar region of the nucleolus, including both the dense fibrillar and the fibrillar center regions. Fibrillarin is also expressed in HeLa cells, 3T3 cells, and human peripheral blood lymphocytes. In metaphase and anaphase, fibrillarin is found on putative chromosomal nucleolar regions (NORs). During telophase, fibrillarin is an early marker for the site of the newly forming nucleolus. The structure of fibrillarin includes an RNA-binding domain and an RNP consensus sequence, which is consistent with the association of fibrillarin with the U3 small nucleolar RNA. Fibrillarin is involved in processing rRNA transcripts in the nucleolus.