Gene therapy for advanced breast cancer is expected to be useful therapeutic approach. Strategies in ongoing clinical protocols can be divided into four: (1) suppression of oncogenes or transduction of tumor suppressor genes; (2) enhancement of immunological response; (3) transduction of suicide genes; (4) protection of bone marrow using drug resistance gene. We have started clinical study of multidrug resistance (mdr1) gene therapy. Advanced breast cancer patients received high dose chemotherapy and autologous peripheral blood stem cell transplantation (PBSCT) with mdr1-transduced hematopoietic cells, and then were treated with docetaxel. 2 patients have been treated so far, and in vivo enrichment of mdr1-transduced cells with docetaxel treatment was seen. Both patients are in complete remission and have no apparent adverse effect from mdr1 gene transduction.