Acidic fibroblast growth factor (aFGF) is a potent inducer of proliferation and differentiation of a variety of cells. The activity of the growth factor is partly transduced into the cells by activation of the specific FGF receptor tyrosine kinase, but an increasing amount of evidence indicates that translocation of the growth factor into the cells and to the nucleus is also required. In the host laboratory a novel method has been developed to test if the growth factor has gained access to the cytosol. By engineering a farnesylation signal onto the C-terminus of the growth factor, it is possible to monitor translocation into cells, since the farnesylation reaction occurs only in the cytosol and nucleus. Transport into cells depends on specific FGF receptors. A number of mutations of such a receptor (FGFR-4) has been made and their abilit,v to support endocytosis and transport to the cytosol and nucleus will be studied.