In the STAT system of proteins, we have determined the solution structure of the N-terminal domain of STAT4, utilizing new NMR methods, and found evidence for a new dimerization interface. Corroborating evidence for this interface is being pursued via investigation of the N-terminal domains (NTDs) of all seven STAT family members. Our studies involve both NMR and biophysical methods. A manuscript describing these studies is being revised for publication. The interactions indicated in the NTD studies have also enabled us to examine the heterodimerization of STAT1 and STAT2 NTDs. These studies are key to understanding the variety of modes of combination among the STAT proteins that can explain the involvement of the seven STAT proteins in literally hundreds of signaling/DNA recognition processes. The studies are being supplemented with further structural determinations for other STAT NTDs. In this reporting year, we have been working to develop new tagging methods and to prepare unique, mixed labeled and tagged species. These efforts will enable the measurement of monomer-specific structural parameters in the STAT-NTD dimers and complexes. We have also collected small-angle X-ray scattering (SAXS) data with the assistance of Dr. Yun-Xing Wang to facilitate these studies of the STAT NTD homo- and hetero-dimers.