HLA-DR is an MHC class II cell surface receptor encoded by the human leukocyte antigen complex on chromosome 6 region 6p21.31. The complex of HLA-DR (Human Leukocyte Antigen - DR isotype) and peptide, generally between 9 and 30 amino acids in length, constitutes a ligand for the T-cell receptor (TCR). HLA (human leukocyte antigens) were originally defined as cell surface antigens that mediate graft-versus-host disease. Identification of these antigens has led to greater success and longevity in organ transplant.