UASt regulatory sequences drive expression of a mutated form of the fusion protein associated with the mouse 'Wld[S]' mutation. The 'Wld[S]' fusion protein is composed of the N-terminal 70 amino acids of Mmus\Ube4b, a unique 18 amino acid domain generated during the fusion event and the full-length sequence of Mmus\Nmnat1. In this transgene, an H24A amino acid replacement has been introduced into the fusion protein sequence (this mutation largely abolishes Mmus\Nmnat1 enzymatic activity).