Migrant thymic precursors do not express Notch ligands. Freshly sorted CD45⁺ precursors obtained from the perithymic and intrathymic regions of the thymus were analyzed for expression of Notch ligands DL1 (A), DL4 (B), Jagged-1 (C), and Jagged-2 (D) by RT-PCR. Note that in comparison to thymic stromal cells that express readily detectable levels of Notch ligands (▦), migrant thymic precursors from the mesenchyme (□) or epithelium (▪) of the E12 thymus were found to express little if any of the Jagged and Delta families of Notch ligands. Data shown are representative of 3 separate experiments and are expressed as mean ± SEM.