A Standardized Teratoma Assay for Pluripotency Figure 1. Kinetics of Teratoma Formation after Transplantation of Various Specific Numbers of Undifferentiated HES-1 Cells. Specific numbers of undifferentiated HES-1 cells were mixed with MMC-treated foreskin fibroblasts (to a total of 16106 cells) and Matrigel, and transplanted s.c. into NOD/SCID mice. The transplanted animals were monitored weekly for the appearance of tumors, and for the progression of tumor size. The endpoint of the experiments was when the tumors reached a size of $ 1 cm3 or 30 weeks after transplantation. (A): Efficiencies of teratoma tumor formation after transplantation of decreasing numbers of undifferentiated HES-1 cells. The trendline is depicted in dotted line. (B): A Kaplan-Meier plot showing the percentage of surviving mice transplanted with decreasing numbers of hESCs, at various time-points (1 W–30 W) during the transplantation experiments. The mice were sacrificed when the tumors reached a volume of $ 1 cm3. (C): The average time interval between transplantation and the detection of tumors. (D): The average volume of the teratomas at the time of animal sacrifice. All data relate to animals that developed teratomas. Data presented as mean 6 SEM. doi:10.1371/journal.pone.0045532.g001 PLOS ONE | www.plosone.org 3 September 2012 | Volume 7 | Issue 9 | e45532