Fig. 2  Variability in β2m fibril structure. (a) Class averages of C-type fibrils. (b) Three class averages of type A fibrils showing the large variation in cross-over spacing. (c) Histogram showing the frequency of cross-over repeats in the classes analysed (85% of the data). The A-type classes are shown in lilac, and the B-type classes are shown in green. (d and e) Cross sections of maps generated from different classes of (d) A-type and (e) B-type fibrils. Scale bar represents 50 nm (a and b). Map widths in (d) and (e) are ∼ 18–20 nm.