From a functional point of view, S727 phosphorylation is mainly important for STAT3-dependent cellular survival and proliferation [43] modulating key targets such as Socs2, NanoG, and Klf4. In correlation to these assigned functions, it is not surprising that S727 phosphorylation is involved in v-Src-mediated transformation [38,39] and in c-Src/p42/p44 MAPK pathway-dependent breast cancer onset [26]. Here, STAT3 is linked to breast cancer.