Transcriptional regulator that plays a central role in Notch signaling, a signaling pathway involved in cell-cell communication that regulates a broad spectrum of cell-fate determinations. Acts as a transcriptional repressor when it is not associated with Notch proteins. When associated with some Notch protein, it acts as a transcriptional activator that activates transcription of Notch target genes (By similarity). Interacts with activated Notch proteins. Belongs to the Su(H) family.