Therapeutic cloning, whereby embryonic stem cells (ESCs) are derived from patient-specific cloned blastocysts via somatic cell nuclear transfer (SCNT), holds great promise for treating many human diseases using regenerative medicine. Teratoma formation and germline transmission have been used to confirm the pluripotency of mouse stem cells, but human embryonic stem cells (hESCs) have not been proven to be fully pluripotent owing to the ethical impossibility of testing for germ line transmission, which would be the strongest evidence for full pluripotency. Therefore, formation of differentiated cells from the three somatic germ layers within a teratoma is taken as the best indicator of pluripotency in hESC lines. The possibility that these lines lack full multi-or pluripotency has not yet been evaluated. In this study, we established 16 mouse ESC lines, including 3 genetically defective nuclear transfer- ESC (ntESC) lines derived from SCNT blastocysts of infertile hermaphrodite F1 mice and 13 ntESC lines derived from SCNT blastocysts of normal F1 mice. We found that the defective ntESCs expressed all in vitro markers of pluripotency and could form teratomas that included derivatives from all three germ layers, but could not be transmitted via the germ line, in contrast with normal ntESCs. Our results indicate that teratoma formation assays with hESCs might be an insufficient standard to assess full pluripotency, although they do define multipotency to some degree. More rigorous standards are required to assess the safety of hESCs for therapeutic cloning. Therapeutic cloning, whereby embryonic stem cells (ESCs) are derived from somatic cell nuclear transfer (SCNT), holds great promise for treating many human diseases using regenerative medicine. Teratoma formation and germline transmission have been used to confirm the pluripotency of mouse stem cells, but human embryonic stem cells (hESCs) have not been proven to be fully pluripotent due to the ethical impossibility of testing for germ line transmission, which would be the strongest evidence for full pluripotency. Therefore, formation of differentiated cells from the three somatic germ layers within a teratoma is taken as the best indicator of pluripotency in hESC lines. The possibility that these lines lack full multi-or pluripotency has not yet been evaluated. In this study, we established 16 mouse ESC lines, including 3 genetically defective nuclear transfer-ESC (ntESC) lines derived from SCNT blastocysts of infertile hermaphrodite F1 mice and 13 ntESC lines derived from SCNT blastocysts of normal F1 mice. We found that the defective ntESCs expressed all in vitro markers of pluripotency and could form teratomas that included derivatives from all three germ layers, but could not be transmitted via the germ line, in contrasts with normal ntESCs. Our results indicate that teratoma formation assays with hESCs might be insufficient enough standard to assess full pluripotency, although they do define multipotency to some degree. More rigorous standards are required to assess the safety of hESCs for therapeutic cloning.