Geron Corp. began the first FDA-approved embryonic stem cell (ESC) therapy last October, in patients with spinal cord injuries. Now a second company has begun its own clinical trials. Advanced Cell Technology partnered with a UCLA ophthalmologist to inject the eyes of two legally blind patients with about 50,000 embryonic stem cells apiece on July 12.

Given the novelty of the technology, though, it's too early to know whether either company's trials will avoid development of the tumors that ESC therapy sometimes causes—or whether the treatments will have any healing effect. Geron reports that its spinal cord treatment has proved safe so far, but hasn't said whether the patients' conditions improved.

Commenting on the trials, the head of the National Institutes of Health, Francis Collins—who is an evangelical Christian and an enthusiastic supporter of ESC research—said the public shouldn't set its hopes too high: "People need to be prepared for the fact that in this area, it's not going to be successful from day one."