Fig. 4  Three-dimensional reconstructions of the type A and type B forms of β2m fibrils. Side views of an A-type fibril (a) and a B-type fibril (b). The maps are contoured at a volume corresponding to an MPL of 53 kDa/nm. One dimeric density unit is indicated by a red box in (b). The directions of the half-fibrils are indicated by arrows below the maps. Cross sections of the A-type (c) and B-type fibrils (d) show that the structures are formed of crescent-shaped units stacked back-to-back. (e) Superposed contour plots of the A (lilac) and B (green) repeat units, showing that the two fibril types have the same underlying organisation that differs only in the orientation of the two stacks, either parallel or anti-parallel.