For therapeutic applications of small interfering RNA (siRNA), serum stability, enhanced cellular uptake, and facile endosome escape are key issues for designing carriers. In this study, green fluorescent protein (GFP) siRNA was conjugated to a six-arm polyethylene glycol (PEG) derivative via a reducible disulfide linkage (6PEG-siRNA). The 6PEG-siRNA conjugate was also functionalized with a cell penetrating peptide, Hph1 to enhance its cellular uptake property (6PEG-siRNA-Hph1). The 6PEG-siRNA-Hph1 conjugate was electrostatically complexed with cationic self-crosslinked fusogenic KALA peptide (cl-KALA) to form multifunctional polyelectrolyte complex micelles for gene silencing. The resultant siRNA complex formulation with multiple PEG chains showed superior physical stability and resistance to enzymatic degradation. The 6PEG-siRNA-Hph1/cl-KALA complexes exhibited enhanced GFP gene silencing efficiency,for MDA-MB-435 cells in the serum containing condition. The current reducible and multifunctional polyelectrolyte complex micelles are expected to have high potential for efficient delivery of therapeutic siRNA. (C) 2009 American Institute of Chemical Engineers Biotechnol. Prog., 26: 57-63, 2010