Schematic representation of the human IL-3/IL-5/GM-CSFR β chain gene and the remains of a βc pseudogene. (A) The βc chain gene spans 25 kb and is divided into 14 exons, while of the pseudogene, only a part of the promoter and exons 2, 3, and 4 are identified (p-prom, p-2, p-3, and p-4). Coding regions are shown in black, noncoding are shown regions in gray. The phage, cosmid, and BAC clones used to characterize the locus are also indicated. (B) Alignment of the homologous regions of the βc chain gene and the pseudogene. The upper strand represents the βc chain sequence (numbers are relative to the transcription start site); the lower strand represents the pseudogene sequence (numbers are according to BAC clone F45C1). The translation start site is boxed (exon2). The p-promoter region, p-exon2, p-exon3, and p-exon4 have homology of 80%, 85%, 95%, and 89% with the βc gene, respectively.