Mutated sequence from the Mmus\Rbpj gene (Su(H) ortholog) has been reintroduced into the attP site present in TI{TI}Su(H)^attP, resulting in a transcription unit in which endogenous Su(H) regulatory sequences drive expression of the first exon and intron of Su(H) (this encodes 128 amino acids that lie mostly within the non-conserved N-terminal part of Su(H)) followed by mutated Mmus\Rbpj coding region starting at position V81 in the Mmus\Rbpj sequence and containing mutations in three conserved Leu residues (amino acid replacements L386A, L397A, and L466A) which are equivalent to those in the Su(H) ortholog that mediate contact with H protein. As a result of the engineering event, a residual attR site is present in the first intron, and most of this intron is duplicated.