Selection of the Regimen for High-Dose Immunosuppressive Therapy Cyclophosphamide with ATG (Cy/ATG) is a common conditioning regimen with two decades of experience in the treatment of aplastic anemia and has been used safely without reported mortality in the treatment of autoimmune diseases such as systemic lupus erythematosus and rheumatoid arthritis. Cy/ATG not only has less acute toxicity, it has less chronic side effects. Cy/ATG is not associated with late malignancies or cataracts. Cyclophosphamide and anti-thymocyte globulin (horse or rabbit ATG) and rituximab are potent immunosuppressive agents. Rituximab and ATG contribute additional immunosuppression without additional cytotoxicity. Rituximab and ATG given shortly pre-transplant will contribute to the elimination of host T lymphocytes that survive cyclophosphamide or that contaminate the CD34+selected graft. SLE, an autoimmune disease responsive to cyclophosphamide, responds well to a Cy/ATG/rituximab conditioning regimen. For these reasons, Cy/ATG/rituximab will be the conditioning regimen utilized in this study.