Clusterin (apolipoprotein J) is a 75-80 kDa disulfide-linked heterodimeric protein associated with the clearance of cellular debris and apoptosis. In humans, clusterin is encoded by the CLU gene on chromosome 8. CLU is a molecular chaperone responsible for aiding protein folding of secreted proteins, and its three isoforms have been differentially implicated in pro- or antiapoptotic processes. Through this function, CLU is involved in many diseases related to oxidative stress, including neurodegenerative diseases, cancers, inflammatory diseases, and aging. Structure The CLU gene contains nine exons and expresses three isoforms alternatively-spliced at the first exon. The encoded protein isoforms all localize to different subcellular compartments: one isoform localizes to the nucleus; a second isoform localizes to the cytoplasm; and the third is secreted from the cell. They also perform opposing functions: the nuclear CLU binds Ku70 to release BAX and induce apoptosis, whereas the cytosolic and secretory isoforms inhibit apoptosis. The nuclear isoform encodes a 49 kDa protein, while the secretory isoform, which is the main gene transcript, encodes a 75–80 kDa protein after maturation (glycosylation, secretion, and dimerization). The mature protein is a 449-residue, heterodimeric, disulfide-linked glycoprotein composed of two subunits of ~34-40 kDa α- and β-chains. Function Clusterin was first identified in ram rete testis fluid where it showed signs of clustering with rat sertoli cells and erythrocytes, hence its name. CLU is a member of the small heat shock protein family and, thus, a molecular chaperone. Unlike most other chaperone proteins, which aid intracellular proteins, CLU is a Golgi chaperone that facilitates the folding of secreted proteins in an ATP-independent way. The gene is highly conserved in species, and the protein is widely distributed in many tissues and organs, where it participates in a number of biological processes, including lipid transport, membrane recycling, cell adhesion, programmed cell death, and complement-mediated cell lysis. Overexpression of the secretory CLU isoform protects the cell from apoptosis induced by cellular stress, such as chemotherapy, radiotherapy, or androgen/estrogen depletion. CLU promotes cell survival by a number of means, including inhibition of BAX on the mitochondrial membrane, activation of the phosphatidylinositol 3-kinase/protein kinase B pathway, modulation of extracellularular signal-regulated kinase (ERK) 1/2 signaling and matrix metallopeptidase-9 expression, promotion of angiogenesis, and mediation of the nuclear factor kappa B (NF-κB) pathway. Meanwhile, its downregulation allows for p53 activation, which then skews the proapoptotic:antiapoptotic ratio of present Bcl-2 family members, resulting in mitochondrial dysfunction and cell