The pluripotent stem cells [have the] ability to proliferate rapidly and infinitely. But it’s a double-edged sword. After multiple cell cycles, the chances of mutations increases. This could include mutation to produce an oncogene that can cause cancer.

So these treatments are now on hold?

Yes. We are developing allogenic stem cell lines — stem cells from donors. They would not be the patient’s own, but compatible cells to transplant into the patient, much like blood transfusions with compatible blood types.

We are performing rigorous quality tests, including sequencing the stem cells’ genomes to be sure the cells are free from cancer-causing mutations. We perform tests on adult retinal cells generated from these stem cells to assure that they function as normal retinal cells, and those cells are transplanted into mice or rats for a year to assure they are safe.

That’s very different from the way stem cell treatments were originally described to the public. It was going to be “personalized” medicine — using the patient’s own stem cells to generate the adult cells without risk of rejection.

Well, we realized that it would take a great deal of time and would be unrealistically expensive to carry out the deep sequencing and animal studies for each patient’s cells.

How many compatible donor cell lines do you expect will be needed to cover the Japanese population?

Not that many. One particular line — just one — can work for 17 percent of the Japanese population. We estimate that altogether about 100 lines will suffice for the 100 million people in Japan.

How many lines would be needed for the more diverse United States population?

We would need only about 200 lines.