Methods were developed to process diffraction data from epitaxially tw
inned crystals. Four programs for data reduction and two display progr
ams were developed to augment the data-reduction program XDS [Kabsch (
1988). J. Appl. Cryst. 21, 916-924]. The programs can be generalized f
or use with other data-reduction software that provides the user with
a list of the reflections used to determine lattice constants and crys
tal orientation. LATTICE_VIEW generates a PDB file containing 'water m
olecules' at the reciprocal-space coordinates of the strong spots foun
d in the initial data frames. The PDB file is visualized to identify s
pots that belong to the same lattice, obtain unit-cell dimensions for
a lattice, and assess data quality. VECTOR_MATCH is used to find addit
ional spots belonging to a lattice. ACCOUNT4 determines which spots ha
ve been processed by XDS. COMFORT discards reflections that are too cl
ose to a reflection in another lattice. The display programs provide u
seful visual information on the quality of the crystal orientations us
ed. Data with an R(merge) of 7.1% at 2.4 Angstrom resolution were obta
ined from epitaxially twinned crystals of an RNA dodecamer. The data w
ere of sufficient quality to solve the structure with a combination of
molecular replacement and single isomorphous replacement methods.