Affinity tags can reduce merohedral twinning of membrane protein crystals. - Backmark, Anna; Nyblom, Maria; Törnroth-Horsefield, Susanna; Kosinska-Eriksson, Urszula; Nordén, Kristina; Fellert, Maria; Kjellbom, Per; Johanson, Urban; Hedfalk, Kristina; Lindkvist-Petersson, Karin; Neutze, Richard; Horsefield, Rob (Acta Crystallographica: Section D (Wiley-Blackwell), 2008)