52 The Dream of Elixir Vitae CELL THERAPY Gene therapy and RNAi are limited in the number of genes they can affect in cells. One way to overcome this limitation is by replacing the cells themselves, a process known as ‘cell therapy’. Since there are few theoretical restrictions as to the number of genetic modifications cells can endure, cell ther- apy has a greater ‘bandwidth’. For example, in an experiment aimed  at  treating  the  immunodeficiency  disease  SCID-X1, cells from the immune system were extracted from a patient, genetically engineered, and inserted back again with encour- aging results. [19] One growing area involves stem cells. A stem cell is a sort of  ‘unprogrammed’  cell  that  has  the  potential  to  become any  type  of  cell  in  the  adult  body.  Aging  has  been  linked to an age-related inability of stem cells to replenish mature cells and so therapeutic interventions that enhance stem cell functional capacity might ameliorate the age-associated atro- phies of several organ systems. [20] More importantly, with nuclear transfer experiments such as Dolly [3], it is now pos- sible to ‘turn back the clock’ and generate embryonic stem cells from an adult. [21;22] In theory, it is possible to geneti- cally modify these cells according to needs, differentiate them into the necessary tissue or organ and then implant them to treat age-related diseases, a procedure called ‘therapeutic clon- ing’. [23;24] Since these cells are genetically identical to the patient’s there are few or no problems of immune rejection. The ability of stem cells to regenerate virtually all types of tissues holds great promise. [25] In theory, it is possible to create practically all components of a human being in the lab and then replace the patient’s organs and tissues one by one. Stem cells have been used with success against heart disease, [26] or to repair damage to the brain [27] and spinal cord. [28] Also, stem cells are incredibly versatile: transplantation of