v Preface Since its original identification and isolation nearly 20 years ago, trans- forming growth factor-beta (TGF ) has been extensively investigated with respect to its ability to induce and modulate numerous physiological responses in vitro and in vivo. However, attempts to analyze and identify the signal trans- duction pathway(s) utilized by TGF were, for the most part, fruitless and remained elusive until the last decade when the molecule began to emerge from its black box. The isolation and identification of the TGF receptors, as well as the intracellular mediatory SMAD proteins, have allowed for the biochemical characerization of at least some of the pathways involved in TGF signaling. As a result, many new methodologies and techniques are applicable to the study of TGF that until recently were a luxury reserved only for other growth factors and cytokines. It is our aim in Transforming Growth Factor-Beta Pro- tocols to encompass both old and new methodologies into a concise, self- contained volume allowing both experts and novices a benchside reference. The authors contributing the various chapters are all experts in their field and they routinely perform the methodologies described. I thank them for their time and effort. We hope that this protocol manual will become torn and tat- tered from its frequent use on laboratory bench tops. Philip H. Howe, PhD