The expression of the Adenovirus serotype 5 ( Ad5) E1A oncogene sensitizes tumor cells to natural killer ( NK) cell - mediated killing and tumor rejection in vivo. These effects are dependent on the ability of E1A to bind the transcriptional coadaptor protein p300. To test the hypothesis that E1A up- regulates ligands recognized by the NKG2D- activating receptor, we stably transfected the highly tumorigenic mouse fibrosarcoma cell line MCA- 205 with Ad5- E1A or a mutant form of E1A that does not interact with p300 ( E1A- Delta p300). Ad5- E1A, but not E1A- Delta p300, up- regulated the expression of the NKG2D ligand retinoic acid early inducible ( RAE)- 1, but not murine ULBP- like transcript 1, another NKG2D ligand, in four independently derived MCA- 205 transfectants. The up- regulation of RAE- 1 by E1A targeted MCA- 205 tumor cells to lysis by NK cells, resulting in NKG2D- dependent tumor rejection in vivo. Moreover, the up- regulation of NKG2D ligands by E1A was not limited to mouse tumor cells, as E1A also increased the expression of NKG2D ligands on primary baby mouse kidney cells, human MB435S breast cancer cells, and human H4 fibrosarcoma cells.