Arg74 in HLA-DRB1 and DRB3 controls a DR3-related epitope.

TR81 is a specificity closely related to or identical with DR3. In Caucasoids two amino acids, Tyr at position 26 and Arg at position 74 of HLA class II DR beta chains, have been found to be associated with the presence of TR81. Recently, a variant of DRB1*03 identified in American Blacks has been shown to possess Arg at position 74 but Phe at position 26. This codon combination is found to be present in four other cell lines where it still specifies the TR81 determinant. This suggests that the TR81 specificity is uniquely dependent on the presence of Arg at position 74.