Elucidating pathological fibril structure: Amyloid-β (Aβ) is a key pathological contributor to Alzheimer's disease. Gremer et al. used cryoelectron microscopy data to build a high-quality de novo atomic model of Aβ fibrils (see the Perspective by Pospich and Raunser). The complete structure reveals all 42 amino acids (including the entire N terminus) and provides a structural basis for understanding the effect of several disease-causing and disease-preventing mutations. The fibril consists of two intertwined protofilaments with an unexpected dimer interface that is different from those proposed previously. The structure has implications for the mechanism of fibril growth and will be an important stepping stone to rational drug design. Science , this issue p. 116 ; see also p. 45