Two phases of Notch activation occur in lymphoid precursors prior to and after thymus colonization. CD45⁺ precursors obtained from the perithymic (□) and intrathymic (▪) regions were compared with CD45⁺Lin^–IL-7Rα⁺ fetal liver precursors (▦) for evidence of Notch signaling. Note that although all cell preparations were found to express comparable levels of Notch-1 (A) and presenilin (B), precursors from the fetal liver and from the thymic epithelial rudiment, but not the perithymic mesenchyme, show evidence of Notch signaling as indicated by expression of Hes-1 mRNA (D). Data shown are typical of 3 separate experiments and are expressed as mean ± SEM.