A Phase 1 dose-finding study of Universal Chimeric Antigen Receptor T-cells targeting cluster of differentiation (CD) 123 (UCART123) administered intravenously to patients with relapsed or refractory Blastic Plasmacytoid Dendritic Cell Neoplasm (BPDCN), followed by a dose expansion phase in relapsed or refractory BPDCN patients or newly diagnosed BPDCN patients.