Hematopoietic progenitor expression of MDR1 in vitro. Hematopoietic progenitor colonies, HPP-CFC, from bone marrow transduced with the MDR18.1 retroviral vector were analyzed for expression of the MDR1 RNA using the RT-PCR assay. From a group of 99 colonies, 48 were generated by bone marrow transduced in the presence of cytokines (IL-3, IL-6, IL-11, and SCF ), and 51 were generated by bone marrow transduced in the absence of cytokines. As shown in the graph on the left, both groups were efficiently transduced. The transduction in the presence of cytokines (+) results in 83% of colonies positive for MDR1, and transduction in the absence of added cytokines (−) results in 92% of the colonies scoring positive. After transduction (+ or − cytokines), bone marrow cells were plated in either the multifactor HPP-CFC assay or in CSF-1 alone. Of the 99 colonies analyzed, 81 were generated from the HPP-CFC assay and 18 were generated from the CSF-1 platings; these are compared in the graph on the right. Colonies from the multifactor HPP-CFC assay are multilineage and highly proliferative (<1.0 mm in diameter); 96% of these were positive for expression of MDR1 RNA. The colonies grown in CSF-1 alone were predominantly macrophages, and were seldom greater than 0.5 mm in diameter; 50% of these were positive for expression of the MDR1 RNA. When the results of both transduction methods and both plating assays are combined, the overall transduction rate into hematopoietic progenitors is 88%.