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affinity purification and expression cloning (Bartley et al., 1994; de Sauvage et al., 1994; Kaushansky et al., 1994). The TPO receptor was discovered as the cellular homologue of a retroviral oncogene. Experiments with Friend murine leukemia virus led to the isolation of a mutant virus called myeloproliferative leukemia virus (MPLV) (Wendling et al., 1986). MPLV caused a broad spectrum of myeloid malignancies in mice including erythroid, granulocytic, monocytic, megakaryocytic and mast cell leukemias (Wendling et al., 1989). This recombinant retrovirus carried a novel oncogene, v-mpl, that displayed sequence homology to members of the cytokine receptor superfamily. A rearrangement led to a fusion between sequences coding for the extracellular domain of the retroviral envelope protein and a truncated cytokine receptor, consisting of 40 amino acids of the extracellular domain, the transmembrane domain, and the entire cytoplasmic domain (Souyri et al., 1990). This suggested that signaling through the cyto- plasmic domain of v-mpl was responsible for the proliferation and transformation of hematopoietic progenitors. Since I was interested in megakaryocyte formation and transformation, I began to study the pro- perties of c-mpl, the cellular homologue of v-mpl, which at this time was an orphan receptor molecule. To demonstrate that c-mpl was cap- able of signaling for proliferation, I generated a chimeric receptor con- sisting of the extracellular domain of the interleukin-4 (IL-4) receptor and the transmembrane and cytoplasmic domain of c-mpl. When stimulated by IL-4, the cytoplasmic domain of the c-mpl protein was capable of providing a proliferative signal in factor-dependent cell lines (Skoda et al., 1993). The full-length c-mpl protein and a soluble extra- cellular domain was used by several laboratories to clone the mpl- ligand, which turned out to be the long sought for TPO (Bartley et al., 1994; de Sauvage et al., 1994; Kaushansky et al., 1994). TPO causes thrombocytosis in vivo (Kaushansky et al., 1994) and promotes prolif- eration and maturation of megakaryocyte precursors in vitro (Debili et al., 1995; Kaushansky et al., 1995). TPO is mainly synthesized in the liver and the control of TPO production provides an interesting para- digm for autoregulation (Kuter and Rosenberg, 1995). In support of this model (Figure 1), we found that TPO mRNA was not regulated during thrombocytopenia. Furthermore, isolated mouse platelets absorbed high amounts of bioactive TPO out of TPO-conditioned medium in a 41