The human retinoid X receptor beta (RXRB) gene maps to the major histocompatibility complex (MHC) region, between KE4 and COL11A2, approximately 130-kb centromeric to HLA-DPB1. We have recently reported a new polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) method to detect the G to T single nucleotide polymorphism (SNP) located seven nucleotides after the tenth exon of the RXRB gene, or 3'end+7 position according to existing nomenclature. We also reported strong linkage disequilibrium between the HLA-DPB1*0401 and RXRB+7*T alleles. In the present study, we describe two PCR-RFLP methods to detect additional SNPs in the RXRB gene, T to A, at exon10+378 and A to T at 3'end+140. This new methodology permitted the unambiguous assignment of three distinct SNPs at RXRB exon10+378, 3'end+7 and 3'end+140 to form an "RXRB haplotype." The data generated from this study were used to determine linkage disequilibrium between several MHC markers and the RXRB alleles and haplotypes. Family studies revealed significant linkage disequilibrium between the RXRB alleles and a number of HLA-DPB1 alleles.