Protein crystals and diffraction patterns. (a) Protein crystals produce a diffraction pattern when x rays are passed through them. In x-ray crystallography, the crystal is rotated in all directions, and diffracted x rays are collected by a detector. X rays that pass through the crystal without diffraction are blocked from hitting the detector by a beam stop. (b) The unit cell, or repeating unit in the lattice of a protein crystal, may contain one or more protein molecules.