ESEARCH performed over the past few years, in particular, has made it clear that, in addition to the liver, hepatocyte growth factor (HGF) 1 affects virtually every tissue in the body ranging from the nervous system to the immune and reticuloendothelial systems. Recent findings have also revealed that the biological responses of target cells to HGF are not confined to the induction of cell proliferation and motility per se but include a plethora of effects such as inhibition of cell growth, induction of morphogenesis, stimulation of T cell adhesion to endothelium and migration, enhancement of neuron survival, and regulation of erythroid differentiation. Furthermore, the discoveries of the HGF receptor as Met, of an HGF-related factor, called HGF-like protein, and of HGF-like's receptor as Ron, a transmembrane tyrosine kinase similar to the HGF receptor, have added yet more levels of complexity to the nature of HGF and its now growing family.