Proteins used in SPR assays were expressed and purified as described previously . Protein used in DEL selection and crystallography: DNA fragments encoding the WDR91 WDR domain (P392-A747) were PCR-amplified from cDNA (MGC: 29694). Each fragment was cloned into the pFBOH-MHL expression vector (Addgene Plasmid #162266). The baculovirus expression vector system (BEVS) was used to express these recombinant proteins. The harvested cells were lysed by sonication in a buder containing 20 mM Tris-HCl pH 8.0, 500 mM NaCl, 5% glycerol, 0.06% NP40, benzonase, and protease inhibitor cocktail. The protein was initially adinity purified by TALON Metal Adinity Resin (Cat# 635504 Clontech) and eluted with 20 mM Tris-HCl pH 8.0, 500 mM NaCl, 5% glycerol and 250 imidazole. The eluted fractions were then further purified by size-exclusion chromatography (SEC) on a Superdex200 26/60 column equilibrated with a buder containing 20 mM Tris-HCl pH 8.0 and 150 mM NaCl. The combined peak fractions from SEC were further purified by Ion exchange chromatography using Source 30Q column, equilibrated with 20 mM Tris-HCl pH 7.5 and eluted with a buder containing 20 mM Tris-HCl pH 7.5, and 1M NaCl gradient. The protein purity and quality were assessed by SDS-PAGE and mass spectrometry. The pure fractions were pooled together and concentrated to 10.9 mg/mL, flash frozen in liquid nitrogen, and stored in a -80 °C freezer for future crystallization trials.