NEDD8 ultimate buster 1 (NUB1) is a negative regulator of the NEDD8 conjugation system. NUB1 interacts with NEDD8 and another ubiquitin-like modifier, UBD (also known as FAT10) to promote their degradation and that of their conjugated proteins (Kito et al, 2001; Kamitani et al, 2001; Hipp et al, 2004; Schmidtke et al, 2006). NUB1 interacts directly with both NEDD8/UBD and with PMSD4, a subunit of the 19S cap of the 26S proteasome, through a ubiquitin-like domain (UBL), and in this way promotes the contact between the proteasome and its substrate (Kito et al, 2001; Kamitani et al, 2001; Hipp et al, 2004; Tanji et al, 2005; Schmidtke et al, 2006; reviewed in Tanaka et al, 2012; Schmidtke et al, 2014). There are two isoforms of NUB1 in human cells that differ by the presence of a 14 amino acid insertion in the NUB1L. NUB1L promotes the degradation of NEDD8 more efficiently than the short isoform (Tanaka et al, 2003).