Ubiquitin-like molecules show sequence and structural similarity to ubiquitin. Unlike ubiquitination, modification by UbLs usually does not target proteins for destruction by the proteasome. A notable exception may be FAT10, a modifier that serves as a ubiquitin-independent signal for proteasomal degradation [8] . The conjugation of UbLs to target proteins follows reaction pathways similar to those involved in ubiquitination [9] . The enzymes that attach or cleave UbLs are generally distinct from the ligases or proteases of the ubiquitin pathway.