Acts as a transcriptional coactivator for NOTCH proteins. Has been shown to amplify NOTCH-induced transcription of HES1. Interacts through its N-terminal region with the ankyrin repeat region of the Notch proteins NOTCH1, NOTCH2, NOTCH3 and NOTCH4. Forms a DNA-binding complex with Notch proteins and RBPSUH/RBP-J kappa. Nuclear, in a punctate manner. The C-terminal domain is required for transcriptional activation. Belongs to the mastermind family. Extended N-terminus. Extended N-terminus.