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The process by which an individual's repertoire of unique antibody-forming cell precursor (B-cell)' specificities (clonotypes) is acquired has been the subject of extensive investigation and controversy over the past several years (1-3) . To help clarify the mechanism by which diversification of B-cell clonotypes might occur, this laboratory has conducted an investigation of the B-cell clonotypes available during the early stages of neonatal development, i .e ., at an important period in the initial development of the specificity repertoire .Previous reports from this laboratory have demonstrated that the adoptive transfer of neonatal spleen cells to an adult, carrier-primed environment maximizes the responsiveness of neonatal B cells (4-6) . The principle rationale for this approach-that neonatal B cells are indeed immunologically competent when provided with ancillary mechanisms by the adult host environment-has been verified by other investigators (7,8) . Previous analyses from this laboratory have also shown the similarity of neonatal and adult primary B cells, in terms of the kinetics and antigen dose dependence of the antibody response (4), and the relative ease of hapten inhibition of primary B-cell stimulation (6,9,10) . Recently, an analysis of the frequency of neonatal B-cell precursors specific for the haptenic determinants 2,4-dinitrophenyl (DNP) ; 2,4,6-trinitrophenyl (TNP) ; and fluorescein (FL) has demonstrated that a disparity exists in the rate of development of FL-specific precursor cells, as compared to DNP-or TNP-specific cells (5) . Thus, at an early stage in the generation of specificity, when the total number of available specificities is limited, significant disparities may exist due to potential "all or none" expressions .This report extends these observations by an analysis of the clonotypes available in neonatal BALB/c mice which are specific for the DNP and TNP haptenic *

Maul et al. show that proteins associated with variable gene hypermutation, such as Spt5 and AID, are recruited to the initiating form of RNA polymerase II specifically in cells activated in germinal centers but not in culture.

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