HLA-B51 (B51) is an HLA-B serotype. The serotype identifies the more common HLA-B*51 gene products. B51 is a split antigen of the broad antigen B5, and is a sister serotype of B52. There are many alleles within the B*51 allele group. B51 is associated with several diseases, including Behçet's disease. Behçet disease is a vasculitis disorder defined by the presence of recurrent oral ulcers, genital ulcers, eye disease (characteristically uveitis), and skin lesions. Risk of developing the disease is associated with being a carrier of the HLA-B51 allele.