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