The SASSIE modelling workflow for monomeric human IgA1 (Hui et al., 2015). This work and that in Fig. 5 was presented at the 16th International Conference on Small-Angle Scattering at the Technische Universität Berlin, Germany, on 13–18 September 2015. (a) The goodness-of-fit R factors for the calculated I(Q) curves from 146 484 hydrated IgA1 structures were calculated relative to the I(Q) curve extrapolated to zero concentration. The R factors were plotted against the RG value calculated for each hydrated model. The experimental RG value of 5.77 ± 0.04 nm (unless otherwise stated, uncertainties are reported as one standard deviation) is shown by the vertical blue line, and a coloured band indicates the ±10% range of X-ray RG values used for filtering the best-fit models. The inset shows an expanded view for the R factors below 10%. (b) The SAXS curve fit for the median best-fit structure for full-length IgA1 identified from a cluster of 112 best-fit structures (Figs. 1 and 5). The calculated I(Q) and P(r) curves are shown in red and compared with the experimental data in black. (c) The SANS fit for the unhydrated structure corresponding to the best-fit SAXS hydrated structure is also shown.