The following sections show how the 26-10 Fab fragment complexed with digoxin was solved by generalized molecular replacement (Brünger 1991c). The space group of the 26-10 Fab/digoxin crystals is (Strong 1990), with the b-axis unique, and a non-crystallographic twofold symmetry is present. The following strategy was employed to solve the structure:

- Self-rotation function.
- Modification of the elbow angle of a known Fab structure.
- Cross-rotation function with the modified Fab structure.
- Filtering the rotation function by
**PC**-refinement. - Analysis of the
**PC**-refinement. - Translation function for molecule A, using the
**PC**-refined model. - Translation function for molecule B.
- Combined translation function to determine the relative position between A,B.
- Rigid-body refinement.

**Figure 17.1:** Overview of molecular replacement.

- Self-rotation Function
- Modification of the Elbow Angle of a Known Fab Structure
- Cross-Rotation Function with the Modified Fab Structure
- PC-Refinement of the Highest Peaks of the Cross-Rotation Function
- Analysis of the PC-refinement
- Translation Function for Molecule A Using the PC-refined Model
- Translation Function for Molecule B
- Combined Translation Function to Determine the Relative Position between A and B
- Rigid-Body Refinement