Signals from Jagged1 but not Delta1 are completely blocked by γ-secretase inhibitor X (inhibitor X). DN1 thymocytes were plated at 2000 cells/well on OP9-MigR1, OP9-Jagged1, or OP9-Delta1 in the presence of carrier (0.1% DMSO) or increasing concentrations of inhibitor X. After 12 days coculture, the cells were counted and examined for expression of differentiation markers. (A) Recovery of DN1 thymocytes per well in the presence of carrier alone (□), 1.0 μM (▦), or 3.0 μM (▪) inhibitor X. (B) Inhibition of B-cell development monitored by expression of CD19. (C) Maturation through the DN stages monitored by expression of CD44 and CD25.