The major and a minor class II beta-chain (B-LB) gene flank the Tapasin gene in the B-F/B-L region of the chicken major histocompatibility complex

We have identified the major histocompatibility complex class II beta-chain (B-LB) genes present in the B-F/B-L region of the B complex of nine well-c haracterized lines of chickens and have cleared up much of the confusion co ncerning numbers and location of B-LB genes in this region. By amplifying D NA sequences between adjacent genes, we found two B-LB genes that lie on ei ther side of Tapasin. The dominantly expressed 'major' B-LB gene in all hap lotypes lies between Tapasin and RING-3, and belongs to the B-LBII family o f class II beta-chain genes. The poorly expressed 'minor' B-LB gene in all haplotypes lies between B-lec1 and Tapasin, and belongs either to the B-LBI I family or to the previously unmapped B-LBVI family of class II beta-chain genes. The data suggest that the B-LBII and B-LBVI genes are two lineages of B-LB genes and we propose that they all be termed B-LB genes. The locati on of a third B-LB gene in the B12 haplotype (and possibly other haplotypes as well) has yet to be determined. The structural organization and express ion of the class II beta-chain genes in the B-FIB-L region is similar to th at of chicken class I (B-F) genes, one functional result of which is differ ential resistance to disease and response to vaccines.