Our results support the hypothesis that non-expanded cd45- [cd34+/kdr+]EPCs are juvenile protective factors which change with age, and are capable of attenuating spontaneous ischemic-hemorrhagic stroke progression when infused as freshly isolated juvenile EPCs at ∼4–8×104 EPCs/kg peripheral infusion in older stroke-prone rats prior to the onset of stroke. The gene discussed is PTPRC; the disease is hemorrhagic stroke.