May act as a receptor for annexin II on marrow stromal cells to induce osteoclast formation. Widely expressed. Highly expressed in lymphocytes. Expressed in both resting CD4(+) and CD8(+) T-cells. PubMed:16895901 reports that it is a type I membrane protein. However, no clear transmembrane region is detected by prediction methods, suggesting that its localization at the plasma membrane is unsure.