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The interleukin 7 receptor α-chain (IL-7Rα) is essential for T cell development in both humans and mice and for B cell development in mice. Whereas the transcription factor PU.1 regulates IL-7Rα expression in mouse pro–B cells via a GGAA motif, we demonstrate here that GA binding protein (GABP) bound to this site and was essential in the regulation of IL-7Rα expression in T cells, where PU.1 is not expressed. Moreover, IL-7Rα expression was diminished substantially in thymocytes but was normal on B220⁺ fetal liver cells from mouse embryos with diminished expression of GABPα. Thus, GABP is essential for the regulation of IL-7Rα expression in T cells, and the differential regulation of IL-7Rα in distinct lymphoid lineages is achieved at least in part by differential recruitment of factors to the same GGAA motif.