their recognition of antigen almost exclusively in the context of self major histocompatibility complex products, unlike B cells, which recognize an­ tigen alone. It is this recognition, mediated by the T-cell receptor, that sets into motion the diverse cell-cell interactions, which control the differentiation and regulation of the immune response. Although its importance was well established, the molecular nature of the T-cell receptor remained elusive for two decades. Many hypotheses as to its structure and precise function were put forward, using immunoglobulin as a basis for conjecture, but "the Holy Grail of Immunology" remained ephemeral until three years ago. In the ensuing years, both immunologists and molecular biologists have contributed to an explosion of data unsurpassed by any previous period in the field.