Definition: In this task, you are given an abstract of article. Your task is to generate title for this article. Preferred titles are under thirty words.
Input: Ubiquitin functions by covalently modifying other proteins. In the past few years, a surprising number of other proteins have been identified that, despite often being only slightly similar to ubiquitin, can also be attached to proteins. Newly discovered parallels between the activation of ubiquitin and the biosynthesis of certain enzyme cofactors now hint at the possible evolutionary origins of the ubiquitin system.

Evolution and function of ubiquitin-like protein-conjugation systems