ENPEP and osteosarcoma: The most important properties of osteosarcoma stem-like/progenitor cells are their capacities for self-renewal and tumorigenesis.24,25 NPJ4+Apa significantly decreased the stem cell frequency in osteosarcoma stem-like/progenitor cell spheroids (Fig. 6c), whereas free drugs did not have any effect, even under the co-treatment.