In addition to tyrosine phosphorylation, STAT1 and STAT3 are phosphorylated at serine-727, which contributes 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 Interleukin-6 regulation of STAT serine phosphorylation is unknown (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).