Recombination signal binding protein Jκ (RBP-Jκ), also designated KBF2 or CBF1, is the mammalian homolog of the Drosophila suppressor of hairless (Su (H)), a protein involved in the development of the peripheral nervous system. RBP-Jκ is ubiquitously expressed in mammalian tissues and is involved in the regulation of gene expression. RBP-Jκ has been shown to directly interact with the intercellular domain of the cell surface receptor Notch 1. Proteolytically cleaved Notch 1 translocates to the nucleus, where it binds DNA-bound RBP-Jκ and activates transcription of target genes. These genes include NFκB p52 and the Epstein-Barr virus (EBV) protein EBNA-2, both of which contain RBP-Jκ-binding sequences within their promoter regions.