We established new human tumor xeno-transplant animal models that are highly efficient for engraftment using unconditional NOD/SCID/γc^null (NOG) mice. These models are clinically and biologically relevant, which are suitable for in vivo studies of patho-physiology and development of treatment strategies against human cancer and viral infection. This review briefly describes the growth behavior and maintenance of   characteristic  features of tumor  cells as well  as  pharmacological  intervention of tumor growth by NF-kB inhibitors in NOG mice. Virus-infected cells and other cancer cells induced a large tumor very rapidly and efficiently with infiltration of the cells in various organs of NOG mice and eventually developed clinical signs of near death such as piloerection, weight loss and cachexia. Importantly, tumor cells grown in NOG mice maintained original histomorphology as well as expression patterns of tumor markers. Tumor cells sustained a strong NF-kB activity in vivo and induced NF-kB components were indistinguishable from those in cells cultured in vitro. Treatment of tumor-bearing mice with NF-kB antagonists apparently retarded the tumor  growth.  The  rapid  and  efficient engraftment of tumor cells in NOG mice suggest that these are very useful   animal   models  which  could  provide  a  novel system  to clarify the molecular mechanism of growth of cancer cells as well as to develop new anticancer therapies.