This model is driven by multiple tandem insertions of a transgene into intron 3 of Grm1 (metabotropic glutamate receptor 1) with concomitant deletion of an intronic sequence that increases expression of Grm1. Homozygous TG3 mice form primary melanoma tumors on pinna and perianal regions, in addition to metastatic tumors in lymph nodes, lung, and liver (39–42). The gene discussed is GRM1; the disease is metastatic neoplasm.