MDM2 and Angelman syndrome: The initial screen targeted cullins 1, 2, 3, 4A and 5, core subunits of a superfamily of multisubunit ubiquitin ligases [18]; APC2, a cullin-related subunit of the anaphase-promoting complex [19]; E6-AP (also known as Ube3A), a single subunit E3 whose mutation underlies a human mental retardation syndrome called Angelman syndrome [20]; and Mdm2, an E3 that was reported to ubiquitinate PSD-95 [21].