High molecular weight biomolecules are becoming more and more important in the development of new therapeutic drugs. However, the hydrophilic nature of such molecules creates a major limitation for their application--poor penetration through biological membranes. In 1994, a new class of peptides--cell-penetrating peptides (CPPs)--was discovered. CPPs seem to greatly facilitate the delivery of hydrophilic macromolecules over the plasma membrane, both in vitro and in vivo, and show promise for therapeutic purposes. One such example--suppressor of cytokine signalling 3 protein--was shown to act as an effective inhibitor of acute inflammation in vivo owing to its successful delivery by CPPs.