English: Chemical structure of the saponin natural product QS-21 (also termed QS21), consisting of a quillaic acid triterpene (black) substituted with a branched trisaccharide (blue) and a linear tetrasaccharide (green), which is in turn connected to an acyl chain (red) via a hydrolytically labile ester (marked with an asterisk). QS-21 is a 65:35 mixture of the Apiose- and the Xylose-substituted variants of above molecule.