Fig. 1  Helical and subunit repeats of β2m amyloid fibrils. (a) Negative-stain EM showing the helical twist in β2m fibrils. One cross-over repeat (180° turn) is labelled. (b) Cryo-EM class average (upper) and reprojection (lower) of the 3D reconstruction for a type A fibril. (c) Cryo-EM class average (upper) and reprojection (lower) for a type B fibril. (d) Negative-stain image of a cross-over with a marked region of subunit repeats. (e) Diffraction pattern from negatively stained fibrils with single sided staining. The marked layer line (arrows) indicates a spacing of 5.88 nm, corresponding to the subunit repeat. The layer line lies on an axis tilted by ∼ 7° from the fibre (vertical) axis, corresponding to the long-range helical twist of the fibrils. The intensity of the strong central diffraction was reduced by a factor of 10 so that it could be displayed together with the subunit layer lines. Scale bars represent 100 nm (a) and 50 nm (b–d).