PLC/PRF/5 cells expressing 3xFlag-SPOP were lysed and incubated with ANTI-FLAG M2 Affinity Gel (Sigma-Aldrich, St. Louis, MO, USA) at 4 °C overnight. The immunoprecipitated complexes were separated by SDS-PAGE and stained with Coomassie Blue. Mass spectrometry assays were performed at Shanghai Applied Protein Technology Co., Ltd.