UAS sequences regulate expression of smo coding sequence in which amino acids 757-915 of the cytoplasmic tail have been replaced by a circularly permutated yellow fluorescent protein (cpYFP) and in which the Ser residues in the SAID domain (positions 667, 670, 673, 687, 690, 693, 740, 743 and 746) have been replaced by nonphosphorylatable Ala residues.