Genetic abnormalities leading to immunodeficiency disorders can be corrected by replacing the defective genes. This can be done by bone marrow transplantation where the defective cells are replaced, supplying new cells with functional genes. This has been used in a number of different kinds of SCID. Gene therapy has also been used, collecting defective T cells, inserting the needed gene linked to a retrovirus, and returning the corrected cells to the patient. This has been used to correct adenosine deaminase deficiency in SCID.