The AZUL or amino-terminal zinc-binding domain of ubiquitin E3a ligase is found in eukaryotes, and is an unusual zinc-finger domain. The final cysteine is usually mutated in Angelman syndrome patients. It is likely that AZUL plays a role in Ube3A substrate-recognition [1].

1: Zn-binding AZUL domain of human ubiquitin protein ligase Ube3A. Lemak A, Yee A, Bezsonova I, Dhe-Paganon S, Arrowsmith CH; J Biomol NMR. 2011;51:185-190. PMID:21947926