The tumorigenicity has been evaluated by in vitro assays (e.g.,
soft agar formation assay, karyotype analysis) and/or in vivo assays
(transplantation into immunodeficient animals) [8,9,19,20]. When
these assays are performed to detect a trace amount of tumorigenic
cells in hCTPs, they need to be of high sensitivity. Recently, we
developed a highly sensitive in vivo tumorigenicity test using
severely immunocompromised mice, NOG mice, in combination
with Matrigel. Subcutaneous transplantation into NOG mice with
Matrigel allowed inoculation with 1 107 cells and actually achieved
detection of 0.002% HeLa cells spiked into hMSCs in a half of
the mice (unpublished data). However, in vivo tumorigenicity test