Constitutive activation of signal transducers and activators of transcription (STATs) has been associated with oncogenesis. Previously, a protein required for T-cell transformation by the DNA tumor virus herpesvirus saimiri (HVS) strain 484, designated tyrosine kinase-interacting protein (Tip-484), was shown to interact with and dramatically upregulate the activity of the STATs in an Lck-dependent manner. The minimal region of Tip- 484 responsible for binding Lck was defined as a 10-residue C-terminal Src- related kinase homology domain, an 18-amino-acid spacer, and a 10-residue potential SH3 binding domain. This region is termed the LBD (for Lck binding domain). The present data show that only the LBD of Tip-484 is needed to activate Lck in vitro and in vivo. Finally, the LBD was shown to form a complex with STAT3 in vitro, and expression of the LBD in T cells led to STAT3 activation equal to that of full-length Tip-484. These studies demonstrate that the 48-amino-acid LBD of Tip-484 can perform as effectively as the full-length protein in vitro and in vivo.