When we force-overexpressed REIC/Dkk-3 in cancer cells by using an adenovirus vector that carries the REIC/DKK3 gene (Ad-REIC), an effective suppression of cancer outgrowth was observed in vitro as well as in vivo through not only a direct effect that leads to the selective apoptosis of cancer cells but also an indirect effect relevant to immune activation [4–13, 26–28]. Here, DKK3 is linked to cancer.