IL-7 is a critical factor for the development of B and T cells, acting through its high-affinity receptor (IL-7R). This study investigates the role of IL-7R in lymphoid development by generating mice deficient in the IL-7R gene. These mice exhibit a significant reduction in thymic and peripheral lymphoid cellularity, indicating that IL-7R is essential for early lymphoid development. The mutation affects the expansion of thymocytes before T cell receptor gene rearrangement, highlighting the importance of IL-7R in this phase. Additionally, the study shows that IL-7R is crucial for the proliferation of early B cell progenitors undergoing heavy chain rearrangements. The results suggest that IL-7R is a critical signaling molecule in the early stages of B and T cell development. The study also indicates that the absence of IL-7R leads to impaired lymphoid development, but some development can still occur in the absence of an IL-7R-mediated expansion phase. The findings support the hypothesis that the severity of X-linked immunodeficiency (XSCID) may be partly due to a compromised IL-7/IL-7R interaction. The study also suggests that the IL-7R mutation may affect other receptor systems due to shared subunits. Overall, the study underscores the essential role of IL-7R in the development of B and T cells.IL-7 is a critical factor for the development of B and T cells, acting through its high-affinity receptor (IL-7R). This study investigates the role of IL-7R in lymphoid development by generating mice deficient in the IL-7R gene. These mice exhibit a significant reduction in thymic and peripheral lymphoid cellularity, indicating that IL-7R is essential for early lymphoid development. The mutation affects the expansion of thymocytes before T cell receptor gene rearrangement, highlighting the importance of IL-7R in this phase. Additionally, the study shows that IL-7R is crucial for the proliferation of early B cell progenitors undergoing heavy chain rearrangements. The results suggest that IL-7R is a critical signaling molecule in the early stages of B and T cell development. The study also indicates that the absence of IL-7R leads to impaired lymphoid development, but some development can still occur in the absence of an IL-7R-mediated expansion phase. The findings support the hypothesis that the severity of X-linked immunodeficiency (XSCID) may be partly due to a compromised IL-7/IL-7R interaction. The study also suggests that the IL-7R mutation may affect other receptor systems due to shared subunits. Overall, the study underscores the essential role of IL-7R in the development of B and T cells.