The large-scale production of lymphocytes in the bone marrow reflects a delicate balance between positive and negative regulatory signals. For instance, interleukin 7 (IL-7) provides a positive signal, and appears to be both essential and limiting in the mouse. However, much less is understood concerning the negative molecular signals that may limit the output of lymphocytes. Here, Paul Kincade and colleagues discuss how a chance observation with pregnant mice revealed that sex steroids can act as negative regulators of B lymphopoiesis, and may do so under normal steady-state conditions.
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