Human Insulin-like Growth Factor II (IGFII) contains 67 amino acids and shares similar structural features with IGFI, including a 62% sequence homology. In human plasma, IGFI and IGFII are associated with IGF-binding proteins that transport the polypeptides and partially regulate their actions in vivo. In addition to the insulin receptor, IGFII binds to two forms of IGF receptors, both of which are widely distributed in different tissues and cultured cells. IGFII is mitogenic for a variety of cultured cells, including mouse 3T3 cells,12 normal rat kidney cells, human or chicken fibroblasts and MCF-7 human breast carcinoma cells.