If one gene inappropriately expresses or fails to express a crucial protein at a sensitive point, the result might be fatal.  Yet reprogramming might involve regulating thousands of genes in a process that could involve some randomness.  The ability to produce offspring from cultured cells opens up relatively easy ways to make genetically modified, or transgenic, animals.  Such animals are important for research and can produce medically valuable human proteins.

    A simple chemical treatment can persuade cultured cells to take up a DNA construct.  If these cell are then used as donor for nuclear transfer, the resulting cloned offspring will all carry the construct.  The first transgenic sheep produced this way, Polly, was born in summer of 1997.  Polly and other transgenic clones secrete the human protein in their milk.  These observations suggest that once technique for retrieval of egg cells is precise genetic changes into any mammal and to create multiple individuals bearing the alteration.

    Cloning offers many other possibilities.  One is the generation of genetically modified animal organs that are suitable for transplantation into humans.  At present thousands of patients die every year before a replacement of heart, liver or kidney becomes available.  A normal pig organ would be rapidly destroyed by a "hyperacute" immune reaction if transplanted into human.  The reaction happens because proteins in the pig cells have been modified by an enzyme called alpha-galactosy transferase.  So if the organ from a pig that has been genetically altered so that it lacks this enzyme might be well tolerated if doctors gave the recipient drugs to suppress other, less extreme immune reactions.

    If these animals are made with precised engineered genetic constitution could also be employed more directly in cell based therapies for important illnesses like Parkinson's disease, diabetes and muscular dystrophy.  None of these conditions have currently have any fully effective treatments.  If a couple was willing to produce an embryo that could be treated by advanced forms of gene therapy, nuclei from modified embryonic cells could be transferred to eggs to create children who would be entirely free of a given disease.