2Antibody fragments have great potential as crystallization chaperones for structural biology 3 due to their ability to either stabilise targets, trap certain conformations and/or promote crystal 4 packing. Here we present an example of using a single-chain variable fragment (scFv) to 5 determine the previously unsolved structure of the multidomain protein SP140. This nuclear 6 leukocyte-specific protein contains domains related to chromatin-mediated gene expression 7 and has been implicated in various disease states. The structure of two of the domains (PHD-8 bromodomain) was solved by crystallizing them as a complex with a scFv generated by phage 9 display technology. SP140 maintains a similar overall fold to previous PHD-bromodomains 10 and the scFv CDR loops predominately interact with the PHD, while the framework regions of 11 the scFv makes numerous interactions with the bromodomain. Analysis of our and other 12 complex structures suggest various protein engineering strategies that might be employed to 13 improve the usefulness of scFvs as crystallization chaperones. 14 15