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Ovided a very important tool to facilitate biological study of macrophages [20-23]. Several murine macrophage cell lines from bone marrow [24,25], spleen [26,27], fetal liver [28,29], and lung [30] have been successfully obtained by in vitro infection of primary cell cultures with a recombinant J2 retrovirus carrying the v-raf and v-myc oncogenes. In addition, investigation of the function of both