UASt regulatory sequences drive expression of a Dl-Ser fusion protein which comprises the extracellular and transmembrane domains of Dl and the intracellular domain of Ser. Nucleotides encoding Met619 and Lys620 (at the cytoplasmic face of the transmembrane domain) of the Dl protein are replaced with an XbaI restriction site (which encodes Ser and Arg). The initiating ATG codon of Dl is replaced with a BspH1 restriction site which results in replacing His2 with Asn. The endogenous Dl 5'UTR is replaced with a HindIII-NcoI fragment of the Xenopus β-globin gene leader. The 3'UTR of the Ser cDNA is truncated to 17bp 3' of the stop codon and SV40 polyadenylation sequences are present.