Hyaluronan binding protein 1 (HABP1), also known as C1qBP/C1qR and p32, is a ubiquitous acidic glycoprotein that functions in spermatogenesis and as a receptor for proinflammatory molecules (1, 2). HABP1 is synthesized with a 71 amino acid (aa) N-terminal preproprotein and a 208 aa mature region (3). The 3kD mature mouse HABP1, which contains a MAM33-like sequence, shares 90% and 99% aa sequence identity with human and rat HABP1, respectively. HABP1 assembles into a doughnut shaped trimer, with negatively charged residues asymmetrically distributed on one face lining the channel of the complex (4). HABP1 can be cleaved by cell surface MMP-14/MT1-MMP, a protease important in angiogenesis and tumor metastasis (5). Cell surface HABP1 binds a wide range of extracellular molecules, including hyaluronan, vitronectin, complement component C1q, HMW kininogen, and bacterial and viral proteins (2, 6-9). Within the cell, HABP1 binds to molecules containing the C1q globular domain, multiple isoforms of PKC, mitochondrial Hrk, the cytoplasmic tails of adrenergic and GABA-A receptors, the mRNA splicing factor ASF/SF2, and the CBF transcription factor (10-16). Apoptosis and direct phosphorylation by Erk1/2 induces HABP1 translocation to the nucleus (17).