The c-mpl gene was discovered in 1990 as the cellular homologue of v-mpl, the oncogene of the murine myeloproliferative leukemia virus, MPLV (see Chapter 6) (1). In mice, v-mpl induces a lethal myeloproliferative disease involving multiple hemopoietic progenitors (2–4). In vitro, v-mpl transformed multipotential and lineage-committed progenitors, leading to acquisition of growth-factor independence for both proliferation and terminal maturation (1), These data showed that v-mpl was a potent deregulator of hemopoiesis, but they did not provide any information about the function of the normal c-mpl proto-oncogene.