The class II molecules of the human major histocompatibility complex include the DR, DC, and SB antigens, each composed of an alpha and a beta polypeptide chain. We have isolated a DR beta gene in overlapping cosmid clones made from genomic DNA of a Dw4/DR4 homozygous individual. This gene consists of six exons and spans greater than 20 kilobases. Upon sequencing, it was found to possess several deleterious mutations, each capable of rendering the gene nonfunctional: (i) four splice junctions deviate from the G-T/A-G rule; (ii) two premature termination codons are present in the first domain exon; (iii) a 2-base-pair insertion causes a translational frame shift in the second domain exon. In addition, several amino acid residues that are conserved in all known expressed beta chains have been replaced in the amino acid sequence predicted from the pseudogene. Analysis of the pattern of nucleotide substitutions in the second domain exon suggests that most amino acid replacements occurred after the gene was inactivated. The inactivation may have been caused by insertion of a Kpn I repeat 5' to the promoter region, thereby interfering with transcription of the gene through removal of transcriptional enhancer elements. The DR beta pseudogene seems to be present also in other DR4 individuals.