The past two decades have seen the development of novel treatment modalities for PIDs. These include the use of Ig replacement therapy (for antibody deficiency), bone marrow tramsplantation (for SCID), and gene therapy (for the treatment of X-linked SCID and SCID due to ADA deficiency). In X-linked SCID and in ADA-deficiency gene, corrected bone marrow cells have a selective advantage over unmodified stem cells, contributing to successful engraftment by gene-reconstituted cells. The correction of genetic defects in conditions where the expression of the normal molecule does not provide a selective survival advantage will be more difficult and will require the development of more effective genetic vectors. The elucidation of molecular defects underlying PIDs helps in the development of better methods of diagnosing these disorders and genetic counseling of affected families. The contribution of new genetic techniques for elucidating molecular defects underlying PIDs have been described elsewhere.