A mutant human cDNA coding for the transmembrane and cytoplasmic domains of the receptor was inserted in-frame into mouse genomic sequences. The mutations resulted in alterations of two multifunctional tyrosinase motifs: Y1349VHVNATY1356VNV to Y1349MDMSATY1356MDMS. A floxed neo included in the construct was excised by crossing with Cre deleter mice.