The structure of RU-486 is similar to that of progesterone. It has the same cholesterol backbone, but it lacks a methyl group at the C-19 position and a side chain at the C-17 position. Instead, it has a 4-(dimethylamino)-phenyl group at the 11-beta position. This group is actually perpendicular to the steroid skeleton. It is this 11-beta group that confers antagonistic activities to RU-486. RU-486 also has an extra 1-propynyl chain at the 17 alpha position. It is this side chain that confers increased binding affinity than progesterone for the progesterone receptor.