B-cell antigen receptors are composed of heterodi-mers of heavy (H) and light (L) immunoglobulin (Ig) chains, whereas T-cell antigen receptors (TCRs) are heterodimers of a and/3 or 3 ' and ~ TCR chains. The amino-terminal portion of immunoglobulin and TCR chains has a variable amino acid sequence (V region), whereas the remainder of each chain has a constant amino acid sequence (C region); IgH, IgL, and the various TCR chains comprise separate families en-coded at distinct loci by homologous genes. The V region genes of immunoglobulin and TCR chains are assembled somatically from two (V and [joining] J) or three (V, [diversity] D, and J) germ-line gene s gments depending on the immunoglobulin or TCR family (for review, see Tonegawa 1983; Davis and Bjorkman 1988). Following assembly, transcription of immuno-globulin or TCR genes initiates from a promoter up-stream of the V region gene and terminates down-stream from the associated C gene. High-level expres-sion of rearranged IgH or IgL chain genes depends on the activity of lymphocyte-specific transcriptional en-hancers located in the intervening sequences b tween assembled V and C region genes (for review, see Sen and Baltimore 1989). Similar enhancer elements ap-pear to direct expression of TCR/3 and a genes (Krim-
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Ferrier et al. (1989) studied this question.