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The high-brilliance X-ray beams from undulator sources at third-generation synchrotron facilities are excellent tools for solving crystal structures of important and challenging biological macromolecules and complexes. However, many of the most important structural targets yield crystals that are too small or too inhomogeneous for a `standard' beam from an undulator source, ∼25–50 µm (FWHM) in the vertical and 50–100 µm in the horizontal direction. Although many synchrotron facilities have microfocus beamlines for other applications, this capability for macromolecular crystallography was pioneered at ID-13 of the ESRF. The National Institute of General Medical Sciences and National Cancer Institute Collaborative Access Team (GM/CA-CAT) dual canted undulator beamlines at the APS deliver high-intensity focused beams with a minimum focal size of 20 µm × 65 µm at the sample position. To meet growing user demand for beams to study samples of 10 µm or less, a `mini-beam' apparatus was developed that conditions the focused beam to either 5 µm or 10 µm (FWHM) diameter with high intensity. The mini-beam has a symmetric Gaussian shape in both the horizontal and vertical directions, and reduces the vertical divergence of the focused beam by 25%. Significant reduction in background was achieved by implementation of both forward- and back-scatter guards. A unique triple-collimator apparatus, which has been in routine use on both undulator beamlines since February 2008, allows users to rapidly interchange the focused beam and conditioned mini-beams of two sizes with a single mouse click. The device and the beam are stable over many hours of routine operation. The rapid-exchange capability has greatly facilitated sample screening and resulted in several structures that could not have been obtained with the larger focused beam.

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