Epstein-Barr virus nuclear antigen 2 (EBNA2) and the Notch protein both function within the nucleus as transcriptional adaptor proteins. EBNA2 plays a key role during the immortalization of primary B-cells by Epstein-Barr virus (EBV). Notch proteins are involved in lymphomagenesis as well as in multiple cell fate decisions during tissue differentiation and development. Both, EBNA2 and Notch interact with the DNA binding protein RBP-J and thereby gain access to the promoter of their target genes. In order to identify regions within the J recombination signal sequence binding protein (RBP-J), that are relevant for either the Notch or the EBNA2 interaction, we have performed a mutational analysis of RBP-J. A library of RBP-J mutants was screened by a reverse two-hybrid system for alleles that fail to bind to either EBNA2 or Notch. The sequence analysis of these alleles reveals that a limited and particularly distinct number of amino-acid positions are relevant for either interaction only. Given the important role of RBP-J in B-cell immortalization, the EBNA2/RBP-J protein-protein interaction could be a candidate target for therapeutic intervention in EBV related diseases.