The HLA DR15 haplotype, which is most prevalent among Caucasian MS patients, encodes three functional class II heterodimers, DR15 (the DRA1*0101/DRB1*1501 pair), DRB5 (the DRA1*0101/DRB5*0101 pair), and DQ6 (DQA1*0102/DQB1*0602 pair). The gene discussed is HLA-DRB5; the disease is myeloid sarcoma.