Jagged-1 ligation had no effect on the expression of GM-CSFRα, IL-4R, or Flt-3 in HPCs. Bone marrow–enriched HPCs were cultured with GM-CSF on control or Jagged-1 fibroblasts. After 48 hours or 72 hours of incubation CD45⁺ cells were isolated; RNA was extracted and used in reverse transcription-PCR followed by Southern blotting as described in “Materials and methods.” Lanes 1 and 2, HPCs cultured on 3T3-MSCV (1) or on 3T3-Jagged-1 (2) fibroblasts for 48 hours; lanes 3 and 4, HPCs cultured on 3T3-MSCV (3) or on 3T3-Jagged-1 (4) fibroblasts for 72 hours.