Covalent attachment of dyes to the independent molecules in the crystal. The way in which the components of each crystal drop were prepared is indicated above each image of crystals. In the top row, from left to right, Cy3 has been attached to the A molecule, to both molecules, and to the B molecule. Crystals in all three drops are pink. The bottom row has a similar arrangement of molecules to which Cy5 has been attached, left, A, center, both, and right, B. Crystals in all three drops are blue-green. The center row contains drops with both arrangements of molecules containing both dyes, Cy3 on molecule A and Cy5 on molecule B in the drop on the left, and the opposite arrangement on the right. Both drops are purple in color. The center drop contains a control whose molecules have neither dye attached to them.