LYAR is a 379 amino acid nucleolar protein containing two C2HC-type zinc fingers. Expressed at high levels in immature spermatocytes, early embryos and in fetal liver and thymus with low expression in kidney and spleen, LYAR may function as a novel nucleolar oncoprotein to regulate cell growth. It is suggested that LYAR participates in regulating the stability of C23, a protein that is critical for maintaining the self-renewal and differentiation of embryonic stem cells (ESCs). C23 is a eukaryotic nucleolar phosphoprotein that influences synthesis and maturation of ribosomes. LYAR forms a complex with C23, thereby preventing self-cleavage and maintaining steady levels of C23 in undifferentiated ESCs. Downregulation of LYAR decreases C23 stability in ESCs, which in turn negatively affects growth and increases the rate of apoptosis of these cells.