Upon the completion of their model, Crick and Watson had invited Wilkins to be a co-author of their paper describing the structure. Wilkins turned down this offer, as he had taken no part in building the model. He later expressed regret that greater discussion of co-authorship had not taken place as this might have helped to clarify the contribution the work at King's had made to the discovery. There is no doubt that Franklin's experimental data were used by Crick and Watson to build their model of DNA in 1953. Some, including Maddox, have explained this citation omission by suggesting that it may be a question of circumstance, because it would have been very difficult to cite the unpublished work from the MRC report they had seen.