p62(dok) has been identified as a substrate of many oncogenic tyrosine kinases such as the chronic myelogenous leukemia (CML) chimeric p210(bcr-abl) oncoprotein. It is also phosphorylated upon activation of many receptors and cytoplamic tyrosine kinases. However, the biological functions of p62(dok) in normal cell signaling as well as in p210(bcr-abl) leukemogenesis are as yet not fully understood. Here we show, in hemopoietic and nonhemopoietic cells derived from p62(dok)-(/)- mice, that the loss of p62(dok) results in increased cell proliferation upon growth factor treatment. Moreover, Ras and mitogen-activated protein kinase (MAPK) activation is markedly sustained in p62(dok)-(/)- cells after the removal of growth factor. However, p62(dok) inactivation does not affect DNA damage and growth factor deprivation-induced apoptosis. Furthermore, p62(dok) inactivation causes a significant shortening in the latency of the fatal myeloproliferative disease induced by retroviral-mediated transduction of p210(bcr-abl) in bone marrow cells. These data indicate that p62(dok) acts as a negative regulator of growth factor-induced cell proliferation, at least in part through downregulating Ras/MAPK signaling pathway, and that p62(dok) can oppose leukemogenesis by p210(bcr-abl).