BehÇet's disease (BD) has been known to be strongly associated with the human leukocyte antigen (HLA) B51. This B51 association has been confirmed in many different ethnic groups between the Middle East and Japan, and it has been proposed that BD is prevalent in those ethnic groups along the old Silk Route. The hypothesis could be made that B51 molecules are primarily involved in BD development through specific antigen presentation. However, polymorphic analyses of the TNFB gene and Tau-a microsatellite between the HLA-B and TNF genes indicate that the pathogenic gene of BD is not the HLA-B51 gene itself but another gene located around the HLA-B gene. HLA-C genotyping by the PCR-SSP method also suggests that the BD pathogenic gene is not the HLA-C gene itself but other gene located near the HLA-B gene. Recently we sequenced a single contig of 236,822 bp from the MICA gene (58.2 kb centromeric of HLA-B) to 90.8 kb telomeric of HLA-C and identified 8 novel genes designated NOB1-8 (NOB: new organization associated with HLA-B). During the course of the genomic sequence analysis we clarified the genetic structure of the MICA (MHC class I chain-related gene A) gene and found a triplet repeat microsatellite polymorphism of (GCT/AGC)_n in the transmebrane (TM) region. Furthermore, the microsatellite allele consisting of 6 repetitions of GCT/AGC (MICA A6 allele) was present at a significantly higher frequency in the BD patient group than in the control group and a significant fraction of B51-negative patients were positive for this MICA A6 allele. These results suggest the possibility of a primary association of BD with MICA rather than HLA-B.