In addition to tyrosine phosphorylation, STAT1 and STAT3 undergo serine phosphorylation at serine-727, contributing to maximal transcriptional activity (Wen & Darnell 1997, Shen et al. 2004). Though several candidates exist including Protein kinase C delta (Jain et al. 1999), the kinase responsible for IL-6 regulation of Stat serine phosphorylation has yet to be identified (Jain et al. 1999, Abe et al. 2001, Chung et al. 1997) and the significance of serine phosphorylation is unclear (Decker & Kovarik 2000); STAT3 modifed by serine phosphorylation augments oxidative phosphorylation in mitochondria and supported cellular transformation by oncogenic Ras (Reich 2009).