70 Therapeutic Cloning normal. This procedure, sometimes referred to as telomerase therapy, may indeed one day provide a means of transferring some of the powers of immortal renewal into at least some of the cells of the body. But it has proven difficult to efficiently introduce this, or indeed any gene, into most tissues in the human body. STEM CELLS And so, in the meantime, my mind turned to other ways to  mine  the  rich  vein  of  gold  of  the  immortal  germ-line. One fall day several years earlier, I took a break from work- ing on telomerase and walked along the San Francisco Bay waterfront. I began thinking about what are called stem cells. A stem cell is a cell that can branch like the stems of a tree, either making another stem cell or changing to become a more specialized cell. There are all kinds of stem cells in the body, some more “potent” than others (that is, some have the poten- tial to become more similar in cell type than others do). I wondered that day whether it would be possible to grow a human totipotent stem cell in the laboratory. A human toti- potent stem cell, though entirely theoretical at the time, could potentially branch into any cell in the body. If we imagine the branching of the fertilized egg cell into all the cells in the body, these totipotent stem cells would be analogous to the trunk of the tree of cellular life, the mother of all stem cells. I was well aware of Weismann’s work from my years work- ing on cellular aging, and it occurred to me that if we could isolate and culture such cells from the human germ-line, they might be naturally immortal and telomerase positive, at least until they are directed to become a specific mortal cell type. And, most important of all, all the cells that come from them would be young, just as babies are born young.