Abstract: Objective To prevent teratoma formation by removal of the residual undifferentiated cells from differentiated mouse embryonic stem cells （mESCs） based on PECAM-1 expression, to hopefully promote the safe use of ESCs-based treatment in future. Methods Mouse R1 ESCs were cultured in suspension to form embryoid bodies （EBs） in the absence of leukemia inhibitory factor for 6 days. EBs were then digested into single cells and sorted by magnetic activated cell sorting （MACS） based on PECAM-1 expression. Total of 2×10^6 PECAM-1^＋ and PECAM-1^- cells were injected subcutaneously into nude mice respectively. Teratoma formation was measured after 6-8 weeks. Results Seven out of 8 injected points formed teratoma in the PECAM-1^＋ cells after 8 weeks of injection, with a tumor formation rate of 87.5%. While only 1 out of 8 injected points formed teratoma in the PECAM-1^- cells, with a tumor formation rate of 12.5%. A significantly difference was observed between PECAM-1 positive and negative cells （P=0.01）. Conclusion PECAM-1 is a specific marker for residual cells, which could be utilized to remove residual undifferentiated ESCs from in vitro differentiated ESCs, and eventually solve the tumorigenicity problem of ESCs.

Key words: Embryonic stem cells, Residual, Platelet endothelial cell adhesion molecule-1