Apolipoprotein J (apo J) has been purified from human plasma HDL and characterized by de Silva et al in 1990. Apo J is a 70kD glycoprotein, comprised of two disulfide-linked subunits designated apo Ja (34-36kD) and apo Jβ(36-39kD) . The sequence of the 427 amino acid residues of apo J was deduced by the cDNA cloning and sequencing. The predicted a helical regions of apo J indicated that three of these could generate amphiphilic α helices, and may be lipid - bind domains in apo J . Apo J mRNA was expressed in relatively high levels in brain，ovary，testis and liver .Apo J is unique among previous characterized human apolipoproteins in its structure and tissue distribution. The function of apo J is thought to be involved in a variety of physiological processes, including bind and transport lipids. regulation of complement function and sperm maturation etc.