Effect of cholesterol on HIV gp41 fusion domain-induced lipid mixing (A,B) and release of contents (C,D) between and from liposomes composed POPC:POPS:PI (12:2:1) (A,C) or POPC:POPE:SM:POPS:PI (20:5:2:2:1) (B,D) without (red) and with 33% (purple) cholesterol. 10% of the large unilamellar liposomes were labeled with 1% each of NBD-PE and Rhodamine-PE (A,B) or 16.7% were labeled with 15 mM ANTS and 45 mM DPX (C,D) and the rest were unlabeled. Fusion was induced by the addition of 5 µM HIV gp41 fusion domain at the first sharp rise of each curve. Triton X-100 to give a final concentration of 1% disrupting the liposomes was added at the second sharp rise of each curve.