An area of great controversy in molecular immunology is the mechanism by which the differential expression of secretory and membrane immunoglobulin heavy chain is regulated during B cell development. Since the changes in expression of the two proteins are determined largely by the steady state levels of the mRNAs that encode them, recent work has focused on the regulation of the expression of the two messages. This problem is central to understanding humoral immunity, with the specific antigen driven switch from antibody as receptor to antibody as secreted product and may be of direct relevance to some forms of the common variable immunodeficiency syndrome. In addition, numerous other genes have been shown to be regulated by alternative RNA processing. Since its beginnings, research in immunology has brought about profound changes in our view of biology. Jenner's landmark experiment, inducing a minor illness to prevent a major one, showed that the body's future susceptibility to a particular disease could be manipulated. More recently the demonstration that immunoglobulin V, D, and J gene segments, originally spread over many kilobases (kbs) in the genome, must be assembled to form a functional heavy chain gene has shattered both the concept of a genome fixed at fertilization and the "one gene, one protein" rule. The alternative processing of heavy chain transcripts to produce secretory and membrane forms of immunoglobulin has demonstrated how the same gene can give rise to proteins with alternative structures. Since the discovery of the role of alternative RNA processing in heavy chain mRNA synthesis, numerous other cellular genes have been shown to be regulated by modulation of RNA processing pathways.