NOL1, (synonyms: p120, NSUN1, NOP120), is a 120 kDa proliferating-cell nucleolar antigen and is the most cancer specific of the proliferation-associated nucleolar proteins identified thus far. NOL1 is expressed in G1 and peaks during the early S phase of the cell cycle and it has not been detected in benign tumors and most normal resting tissues. Overexpression of NOL1 caused the transformation of NIH 3T3 cells and expression of an antisense NOL1 construct inhibited the growth of NIH 3T3 cells. NOL is localized in a novel nucleolar microfibrillar structure, and contains, consecutively, four major domains: a basic domain, an acidic domain, a hydrophobic and methionine-rich domain, and a domain rich in cysteine and proline residues. The gene for human NOL1 was assigned to chromosome 12p13.