In the form of gene therapy used successfully for severe combined immunodeficiency syndrome, the genetic engineering of human cells is done by

Severe combined immunodeficiency syndrome is a rare genetic disorder that causes improper function and development of the T and B cells of the immune system. The gene therapy for this disorder is using a genetically engineered retrovirus to insert a healthy gene in the white blood cells of the patient. The white blood cells are then expressing the favourable enzyme that could correct the deficiency.