Signal transducers and activators of transcription (STATs) are latent cytoplasmic transcriptional factors that play an important role in cytokine and growth factor signaling. Here we report a 3.05 angstrom-resolution crystal structure of an Unphosphorylated STAT3 core fragment. The overall monomeric structure is very similar to that of the phosphorylated STAT3 core fragment. However, the dimer interface observed in the Unphosphorylated STAT1 core fragment structure is absent in the STAT3 structure. Solution studies further demonstrate that the core fragment of STAT3 is primarily monomeric. Mutations corresponding to those in STAT1, which lead to disruption of the core fragment interface and prolonged tyrosine phosphorylation, show little or no effect on the tyrosine phosphorylation kinetics of STAT3. These results highlight the Structural and biochemical differences between STAT3 and STAT1, and suggest different regulation mechanisms of these two proteins. (C) 2008 Elsevier Inc. All rights reserved.