UASt regulatory sequences drive expression of the TONDU domain of vg, in which the Cys residue at the 5' end and the Ala residue at position 22 have been replaced with Ser, to make the encoded peptide more polar and improve its solubility. The substitution of the N-terminal Cys also reduces the likelihood of aberrant dimer formation. The resulting peptide has the sequence SVVFTNYSGDTASQVDEHFSRSLNY (the VXXHF motif, essential for interaction with sd protein is left unchanged).