Our most recent studies focus on understanding how aberrant expression of the Raf/MEK/MAP kinase cascade can transform hematopoietic, breast and prostate cells. Raf is a key serine-threonine protein kinase which exerts its effects on the transmission of growth and anti-apoptotic messages. These signals can be initiated after receptor ligation and are transmitted to members of the MAP kinase cascade that subsequently activate transcription factors controlling gene expression. Raf is a member of a multigene family which includes: Raf-1, A-Raf and B-Raf. The roles that individual Raf kinases play in the regulation of normal and malignant hematopoietic cell growth are not clear. Our studies have shown that all three Raf kinases are functionally present in human hematopoietic cells and their aberrant expression can result in abrogation of cytokine-dependency. Differences in the levels of Raf and downstream kinase activities were observed between cytokine-dependent and Raf-responsive Raf:ER-infected cells as estradiol-responsive cells expressed more Raf, MEK and activated MAPK activity than the GM-CSF-dependent Raf:ER-infected dependent cells. Abrogation of cytokine-dependency by the activated Raf:ER proteins was associated with autocrine growth factor synthesis which was sufficient to promote the growth of uninfected TF-1 cells. The expression of the p21Cip1 cell cycle arrest protein is being examined in these cells. Raf has been shown to induce the expression of p21Cip1 which then results in cell cycle arrest. It appears that one of the consequences of transformation to Raf-responsive growth is a lack of induction of p21Cip1. The consequences of the alteration in p21Cip1 expression on the tumorigenicity of the cells is being examined. In summary, these observations indicate that the aberrant expression of certain activated Raf:ER oncoproteins can alter the cytokine-dependency of hematopoietic TF-1 cells. These cells will be useful in evaluating the roles of the individual Raf oncoproteins in cytokine-mediated signal transduction, cell cycle progression, regulation of apoptosis and differentiation. Moreover these Raf-infected cells may be important in the evaluation of the efficacy of novel anticancer drugs designed to inhibit Raf and downstream signal transduction molecules.