Transforming growth factor ? (TGF?) is an acid- and heat-stable 50 amino acid protein originally found in rodents and humans. TGF? is 33% homologous at the amino acid level to epidermal growth factor (EGF). TGF? binds to the EGF receptor, mediates tyrosine phosphorylation of the receptor and promotes anchorage-independent growth of normal rat fibroblasts in soft agar in the presence of transforming growth factor ?. TGF? is secreted by a variety of transformed cells and tumors, embryonic cells and some normal adult cells. TGF? bioactivity has been found in the urine of cancer patients. It has been suggested that it may act as an autocrine growth factor for the induction or maintenance of malignancy.