Construct: sc+t.XSE from which the same sequence has been deleted as in scsisB-2 and in which the following sequences immediately upstream of the sc ORF have been mutated as indicated: CTACCTG -> GTAGGTG ; CTACCTG -> AATAAAC ; CAGGTAG -> CATTTAT (order of these sites is 5' -> 3' with respect to the sc ORF.)