Signaling through the Notch pathway controls cell growth and differentiation in metazoans. Following binding of its ligands, the intracellular part of the cell surface Notch1 receptor (Notch1-IC) is released and translocates to the nucleus, where it alters the function of the DNA-binding transcription factor CBF1/RBP-Jkappa. As a result, CBF1/RBP-Jkappa is converted from a repressor to an activator of gene transcription. Similarly, the Epstein Barr viral oncoprotein EBNA2, which is required for B-cell immortalization, activates genes through CBF1. Moreover, the TAN-1 and int-3 oncogenes represent activated versions of Notch1 and Notch4, respectively. Here, we show that the adenoviral oncoprotein 13S E1A also binds to CBF1/RBP-Jkappa, displaces associated corepressor complexes, and activates CBF1/RBP-Jkappa-dependent gene expression. Our results suggest that the central role of the Notch-CBF1/RBP-Jkappa signaling pathway in cell fate decisions renders it susceptible to pathways of viral replication and oncogenic conversion.