about the involvement of signaling Transforming growth factor in tumor development and metastasis. plays a central role in the signaling network that controls morphogenesis, 2. THE BASICS OF growth and cell differentiation in SIGNALING multicellular organisms. The different members of this pleiotropic family of 2. 1. receptor signaling growth and differentiation factors seem to The family of growth factors regulate many processes in human disease consists of more than thirty members in and, in particular, tumor development. humans alone (15, 16). They cluster in Our understanding of how two major groups, the group composed of initiated signals are mediated has both the bone morphogenetic proteins increased dramatically in the last fifteen (BMP) and growth and differentiation years. Firstly, the prototype of factors (GDFs), and the group formed by this still constantly growing family, was the Activins, and Nodals. The two identified and cloned (1). Secondly, the groups differ in their use of receptors for family receptors were transmembrane receptors and the identified by expression cloning from subsequent activation of the mammalian tissue culture (2-7). Thirdly, transcriptional mediators (for recent genetic screens in Drosophila reviews see (13, 14, 17)).