Protein nucleolar association classes considered. PNAC classifies human proteins into four distinct classes according to their degree of nucleolar association. The nucleolar-enriched protein group (red) consists of proteins that are predominantly nucleolar in all cell types and conditions. The nucleolar-nucleoplasmic group (purple) is composed of proteins identified in both the nucleolus and any other nuclear region. The nucleolar-cytoplasmic group (blue) consists of cytoplasmic proteins that also can localise to the nucleolus. The non-nucleolar group (yellow) comprises all proteins that never localise to the nucleolus. The non-nucleolar proteins can localise to other regions of the nucleus, the cytoplasm, plasma membrane or extracellularly.