Chicken class I1 B gcncs 1130 Chicken major histocompatibility complex class I1 B genes: analysis of interallelic and interlocus sequence variance* Five different chicken B-LB genes were cloned and sequenced. The comparison of these sequences shows that they can be classified as members of two different families, the B-LBII family (containing the B-LBI and B-LBII genes) and the B-LBZZZfamily (containing the B-LBIII, B-LBZY and B-LBVgenes).The extent of polymorphism within each of these families was assessed by in virro amplification of DNA fragments encompassing exon 2 in several haplotypes. The nucleotide sequences were determined, and pairwise relationships were evaluated. In the course of this work, a sixth gene termed B-LBVI was identified, defining a third family (B-LBVZ family). Polymorphism of the B-LBZII or B-LBVZfamilies is far less extensive than that of the B-LBIZfamily. In this latter, the distribution of conserved and polymorphic residues is similar to what has been described in mammals. These families seem to have been generated by gene duplication events giving rise to several isotypes, as observed in mammals. However, phylogenetic analyses indicate that these families are not homologous to their mammalian counterparts. Evaluation of the level of transcription of these different genes showed that genes from the B-LBZZ family are predominantly transcribed over those of the other families.