a, Pol II ubiquitination in the absence of UVSSA or CSB. The experiment was repeated two times independently with similar results. For Western blot source data, see Supplementary Fig. 1. b, Processing tree. Number of particles in a particular class is reported above the density. Densities used for further processing are coloured as in Fig. 3c. c, Final composite map created from the focused refined maps. d–f, Local resolution estimate for the composite map and focused refined maps containing CRL4CSA g, Fourier shell correlation plots for all focused refined maps and the composite map. h, Angular distribution plot for the high-resolution Pol II class used as a starting point for focused classifications.