The protein is the receptor that involved in the costimulatory signal essential for T-lymphocyte proliferation and interleukin-2 production, by binding CD28 or CTLA-4. It may play a critical role in the early events of T-cell activation and costimulation of naive T-cells, such as deciding between immunity and anergy that is made by T-cells within 24 hours after activation. Isoform 2 interferes with the formation of CD86 clusters, and thus acts as a negative regulator of T-cell activation. The protein interacts with MARCH8， human herpesvirus 8 MIR2 protein, adenovirus subgroup B fiber proteins and acts as a receptor for these viruses.It is expressed by activated B-lymphocytes and monocytes and promoted by MARCH8 and results in endocytosis and lysosomal degradation.It contains 1 Ig-like C2-type(immunoglobulin-like) domainand 1 Ig-like V-type (immunoglobulin-like) domain.