UASt regulatory sequences drive expression of a Ser-Dl fusion protein which comprises the extracellular and transmembrane domains of Ser and the intracellular domain of Dl. Nucleotides encoding Tyr1248 and Arg1249 (at the cytoplasmic face of the transmembrane domain) of the Ser protein are replaced with an XbaI restriction site (which encodes Ser and Arg). The initiating ATG codon of Ser is replaced with a BspH1 restriction site which results in replacing Phe2 with Ile. The endogenous Ser 5'UTR is replaced with a HindIII-NcoI fragment of the Xenopus β-globin gene leader. The 3'UTR of the Dl c1 cDNA terminates 242bp 3' to the predicted stop codon and SV40 polyadenylation sequences are present.