The MIH5 monoclonal antibody specifically binds to CD274, also known as B7-H1 or PDL1, a 43-kDa glycoprotein encoded by the Pdcd1lg1 gene of the B7 family of the Ig superfamily. Pdcd1lg1 mRNA is expressed in more tissues than other members of the B7 family; transcripts are found in lymphoid tissues and many, but not all, non-lymphoid tissues. The protein has been detected at low levels on resting peripheral T and B lymphocytes, macrophages, and dendritic cells. B7-H1 mRNA and protein expression are upregulated upon activation of T and B cells, macrophages, dendritic cells, and epidermal keratinocytes by a variety of stimulatory factors. B7-H1's receptor, PD-1, contains an ITIM (Immunoreceptor Tyrosine-based Inhibitory Motif) on its intracytoplasmic region and is expressed on activated B and T lymphocytes, suggesting that B7-H1-PD-1 interaction may be involved in the negative regulation of immune responses. The second PD-1 ligand, B7-DC (PD-L2), is also a member of the B7 family of the Ig superfamily. Furthermore, B7-H1 may participate in positive immunoregulation, or costimulation of T cells, through an additional receptor, which is not PD-1 and distinct from the alternate receptor for B7-DC. The MIH5 antibody blocks the binding of PD-1-Ig to B7-H1 transfectants.