The human beta T cell receptor (TCR) locus, comprising a complex family of genes, has been sequenced, The locus contains two types of coding elements-TCR elements (65 variable gene segments and two clusters of diversity, joining, and constant segments) and eight trypsinogen genes-that constitute 4.6 percent of the DNA. Genome-wide interspersed repeats and locus-specific repeats span 30 and 47 percent, respectively, of the 685-kilobase sequence. A comparison of the germline variable elements with their approximately 300 complementary DNA counterparts reveals marked differential patterns of variable gene expression, the importance of exonuclease activity in generating TCR diversity, and the predominant tendency for only functional variable elements to be present in complementary DNA libraries.