Determining the number of transduced cells required to establish long-term erythrocyte chimerism. Transduced 5-FU-treated marrow cells were injected into mice conditioned with 400 cGy at 2 cell doses, 4 million cells (A) and 2 million cells (B). The percentages of GFP-positive erythrocytes and GFP-positive platelets are displayed. Arrows represent treatment with AP20187 at 10 μg/kg for 3 days by intraperitoneal (IP) injection. Filled symbols represent the GFP-positive erythrocytes and platelets of mice that received AP20187. Open symbols represent mice that did not receive injections of CID. Results indicated that only mice that received 4 million transplanted cells maintained GFP-positive cells and could respond to the CIDs.