The chicken major histocompatibility complex (chicken MHC or B complex), at present the most extensively characterized non-mammalian MHC, differes from its human and rodent counterparts in its overall organization. Prominent features of the B complex are its reduced size, the proximity between class Iα and class IIβ genes, which are intermingled, and the close association of these transplantation antigen-encoding genes with nonclassical MHC genes. In contrast, the genetic organization of individual class Iα and class IIβ genes, including intron-exon structure and certain transcription regulating elements, and the primary structure of their products are remarkably conserved