According to the classical model, phosphorylated Signal transducer and activator of transcription (STAT) monomers associate in an active dimer form, which is stabilized by the reciprocal interactions between a phosphorylated tyrosine residue of one and the SH2 domain of the other (Shuai et al. 1994, Mikita et al. 1996). These dimers then translocate to the nucleus (Akira et al. 1994). Recently an increasing number of studies have demonstrated the existence of STAT dimers in unstimulated cell states, and the capability of STATs to exert biological functions independently of phosphorylation (Braunstein et al. 2003, Li et al. 2008, Santos & Costas-Pereira 2011). As phosphorylation of STATs is not unequivocally required for its subsequent translocation to the nucleus, this event is shown as an uncertain process.