We have previously identified a novel phenotypic dichotomy in breast cancer (BC) based on the response to a SRR2 (Sox2 regulatory region 2) reporter， with reporter responsive (RR) cells being more tumorigenic/stem-like than reporter unresponsive (RU) cells. Since the expression level of Sox2 is comparable between the two cell subsets， we hypothesized that post-translational modifications of Sox2 contribute to their differential reporter response and phenotypic differences. By liquid chromatography-mass spectrometry， we found Sox2 to be phosphorylated in RR but not RU cells. Threonine 116 is an important phosphorylation site， since transfection of the T116A mutant into RR cells significantly decreased the SRR2 reporter luciferase activity and the RR-associated phenotype. Oxidative stress-induced conversion of RU into RR cells was accompanied by Sox2 phosphorylation at T116 and increased Sox2-DNA binding. In a cohort of BC， we found significant correlations between the proportion of tumor cells immuno-reactive with anti-phosphorylated Sox2 and a high tumor grade ( = 0.006)， vascular invasion ( = 0.001) and estrogen receptor expression ( = 0.032). In conclusion， our data suggests that phosphorylation of Sox2 contributes to the tumorigenic/stem-like features in RR cells. Detection of phospho-Sox2 may be useful in identifying a small subset of tumor cells carrying stem-like/tumorigenic features in BC.