Gene  Therapy  for  Severe  Combined
   Immunodeficiency (SCID).

   Normal adenosine deaminase (ADA) gene is cloned and transferred into a
   viral  vector,  which  is then used to infect T cells removed from the
   patient.

   After  growth  in culture, the cells are reimplanted into the patient,
   where  the  normal  ADA  protein  allows  the development of an immune
   response.

   In  the  first  patient  who  tried  this procedure. some of the cells
   (25-30%) incorporate the normal ADA gene.

   Unfortunately, a second child treated a short time later expressed the
   normal  ADA  gene  in  less  than  1%  of  her white blood cells after
   treatment