Ubiquitin is a 77 amino acid protein that targets proteins for degradation by the 26S proteasome. Ubiquitin-like proteins are not directly involved in protein degradation, but appear to have many mechanistic similarities with the ubiquitin pathway. UBL4B (Ubiquitin-like protein 4B) is a 174 amino acid cytoplasmic protein that contains a ubiquitin-like domain. UBL4B likely arose by retroposition during mammalian evolution from UBL4A, an X-linked intron-bearing housekeeping gene. While UBL4A is highly conserved, UBL4B has undergone rapid evolution and may have evolved new functions. Expression of UBL4B is restricted to post-meiotic germ cells in testis and ovarian tissue, where it likely functions in post-translational protein modification.