The transgene comprises the cDNA encoding the human ret proto-oncogene driven by the mouse metallothionein 1 promoter/enhancer and followed by a segment of the simian virus 40 (SV40) genome containing a splice signal, a polyadenylation signal and 3' flanking sequence. Northern blot analysis revealed high levels of transgene expression in tumors and varying levels in melanin-producing cells of other tissues in mice of this line, including skin, muscle and Harderian glands.