Dr. Yamanka’s team began this series of experiments by replacing the retrovirus with an adenoviral vector. While transfections with the genes on separate vectors didn’t work, they did work when the genes were arranged in a specific order on a single vector.
The same arrangement worked when the genes were incorporated into a plasmid.To determine if the plasmid-mediated reprogrammed cells were pluripotent, the scientists transplanted the cells under the skin of immunocompromised mice. The resulting tumors contained a wide variety of cell types from all three germ layers. iPS cells injected into embryos resulted in chimeric mice with the injected cells contributing to almost all cell types.
Previously, Dr. Yamanaka had shown that adult cells can be reprogrammed to become embryonic stem cell like using a cancer-causing oncogene as one of the four genes required to reprogram the cells, and a virus to transfer the genes into the cells. In the last year, Dr. Yamanaka and other labs showed that the oncogene, c-Myc, is not needed.