UASt sequences regulate expression of Delta in which the extracellular domain has been replaced in its entirety by sequence encoding a signal peptide, GFP and the CD4 ectodomain, and the intracellular domain has been mutated so that all 12 Lys residues downstream of the stop transfer peptide have been changed to Arg residues. A single FRT site is present in between the UASt and coding sequences.