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NUMB is a negative regulator of Hh signaling that acts by promoting the ITCH-dependent ubiquitination of GLI1. ITCH is an E3 ligase that is kept in an inactive conformation by an intramolecular interaction between the HECT domain and a WW motif. Binding of the adaptor protein NUMB to the WW region of ITCH displaces the HECT domain and promotes the catalytic activity of the E3 ligase (di Marcotullio et al, 2006; 2011).