Recently, research teams led by Shinya Yamanaka and James Thomson published separate reports that they had genetically modified human skin cells to behave like embryonic stem cells. 1 Like their embryonic counterparts, these induced pluripotent stem cells (iPS cells) were capable of forming all three germ layers both in vitro and in immunode ficient mice, demonstrating their remarkable pluripotential character. Furthermore, the two teams' iPS cell colonies were genetically matched to the human skin cells from which they were derived, thus enlivening the possibility that one day (per haps soon) patient-and disease-specific pluripotent stem cells could be generated for research that could later yield down stream clinical benefits.

Now that human iPS cells have arrived, many will wonder whether the thorny ethical challenges surrounding stem cell research can be fortuitously bypassed. This is hardly the case. While the scientific possibilities of iPS cells are enormously exciting, human iPS cell research raises both new ethical com plexities and old philosophical problems. As colleagues and I have noted elsewhere, it would be a se rious mistake to conclude that iPS cell research averts the need for human embryonic stem cells. 2 Human iPS cell research must proceed together with human embryonic stem cell re search for many important reasons. Ongoing research on human embryonic stem cells is nec essary to inform scientists' growing understanding and analy ses of human iPS cells. Much more work is needed on both