SUMO1-3 (small ubiquitin-related modifiers) are ubiquitin-like proteins that can be conjugated to substrate proteins using a cascade of enzymatic reactions. Protein sumoylation can be reversed by the sentrin-specific (SUMO is also named sentrin) proteases (SENPs). In human cells, SENPs have six members including SENP1, SENP2, SENP3, SENP5, SENP6 and SENP7. SENPs mostly reside in the nucleus. They function to generate mature SUMO1-3 from the SUMO precursors and attenuate signals mediated by sumoylation. The SENP1 catalytic domain comprises amino acid from 419 to 644.