This study investigates the role of interleukin 7 (IL-7) and its high-affinity receptor (IL-7R) in the development of B and T cells. The researchers generated mice genetically deficient in IL-7R and found that these mice displayed a profound reduction in thymic and peripheral lymphoid cellularity. Analysis of lymphoid progenitor populations revealed that the mutation affects early stages of lymphoid development, particularly the phase of thymocyte expansion before the onset of T cell receptor gene rearrangement. The results indicate that the high-affinity IL-7 receptor is critical for the proliferation and expansion of early lymphoid cells, suggesting a nonredundant role in the formation of B and T cell lineages. The study also highlights the importance of IL-7R in the first major phase of thymocyte expansion, which is driven by an unknown factor provided by the thymic microenvironment.This study investigates the role of interleukin 7 (IL-7) and its high-affinity receptor (IL-7R) in the development of B and T cells. The researchers generated mice genetically deficient in IL-7R and found that these mice displayed a profound reduction in thymic and peripheral lymphoid cellularity. Analysis of lymphoid progenitor populations revealed that the mutation affects early stages of lymphoid development, particularly the phase of thymocyte expansion before the onset of T cell receptor gene rearrangement. The results indicate that the high-affinity IL-7 receptor is critical for the proliferation and expansion of early lymphoid cells, suggesting a nonredundant role in the formation of B and T cell lineages. The study also highlights the importance of IL-7R in the first major phase of thymocyte expansion, which is driven by an unknown factor provided by the thymic microenvironment.