The binding of protein transduction domain (PTD)-conjugated proteins to heparan sulfate is an important step in cellular internalization of macromolecules. Here, we studied the pluripotency transcription factor Sox2, with or without the nonaarginine (R9) PTD. Unexpectedly, we observed that Sox2 is strongly adsorbed by heparin and by the fibroblasts without the R9 PTD. However, only the R9Sox2 fusion protein is internalized by the cells. These results collectively show that binding to heparan sulfate is not sufficient for cellular uptake, thereby supporting a recent hypothesis that other proteins play a role in cell internalization of PTD-conjugated proteins. (C) 2012 Elsevier Inc. All rights reserved.