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)).

The purpose of this research monograph is to build up a modern value distribution theory for complex analytic mappings between abstract Riemann surfaces. All results presented herein are new in that, apart from the classical background material in the last chapter, there is no over lapping with any existing monograph on merom orphic functions. Broadly speaking the division of the book is as follows: The Introduction and Chapters I to III deal mainly with the theory of mappings of arbitrary Riemann surfaces as developed by the first named author; Chapter IV, due to Nakai, is devoted to meromorphic functions on parabolic surfaces; Chapter V contains Matsumoto's results on Picard sets; Chapter VI, pre dominantly due to the second named author, presents the so-called nonintegrated forms of the main theorems and includes some joint work by both authors. For a complete list of writers whose results have been discussed we refer to the Author Index."