The most recently described member of the ErbB receptor tyrosine kinase family is ErbB-4. In general, the structure of this receptor and its mechanism of action is similar to that described for ErbB-1. However, significantly less is known about ErbB-4 and there are several novel aspects to its structure, mechanism of action, and biology. This includes the spectrum of ligands that activate ErbB-4, the presence of functionally distinct isoforms, a proteolytic processing pathway to the nucleus, and the capacity to induce a spectrum of cellular responses such as mitogenesis, differentiation, growth inhibition, and survival.