A wild-type human cDNA coding for the transmembrane and cytoplasmic domain of the receptor was inserted in-frame into mouse genomic sequences. The cDNA was followed by a neomycin resistance cassette. A fusion protein consisting of the murine extracellular domain fused to the human transmembrane and cytoplasmic domain is produced from this allele. This allele is a control for Mettm1Cpo and Mettm2Cpo.