A genomic fragment containing wild-type Su(H) sequence has been reintroduced into the attP site present in Su(H)attP, restoring the wild-type Su(H) coding region. Part of the first intron is duplicated compared to wild type, to avoid possible splicing defects due to the presence of the residual attR site. A loxP cassette which includes a w[+] marker is also present in the 3'UTR.