Covalent modification of proteins by small ubiquitin-like modifier (SUMO) regulates diverse cellular processes. While many SUMO substrates are identified through individual efforts, affinity-based approaches followed by mass spectrometry are also used to identify in vivo SUMO substrates in yeast and in mammals. Because of low steady-state levels of sumoylation and biases towards abundant targets, identifying sumoylated proteins in vivo can be challenging. The in vitro expression cloning (IVEC) method for SUMO target identification circumvents these challenges and complements the affinity-based approaches. IVEC allows for immediate validation and analysis of substrates through in vitro reconstitution. Furthermore, this method can be easily adapted to identify substrates of specific SUMO ligases.