Acts as a transcriptional coactivator for NOTCH proteins. Has been shown to amplify NOTCH-induced transcription of HES1. Enhances phosphorylation and proteolytic turnover of the NOTCH intracellular domain in the nucleus through interaction with CDK8. Binds to CREBBP/CBP which promotes nucleosome acetylation at NOTCH enhancers and activates transcription. Induces phosphorylation and localization of CREBBP to nuclear foci. Plays a role in hematopoietic development by regulating NOTCH-mediated lymphoid cell fate decisions. Interacts (via N-terminus) with NOTCH1, NOTCH2, NOTCH3 and NOTCH4 (via ankyrin repeat region). Interacts (via N-terminus) with p53 (via DNA-binding region). Forms a DNA-binding complex with Notch proteins and RBPSUH/RBP-J kappa/CBF1. Also binds CREBBP/CBP and CDK8.Forms a complex with PRAG1, NOTCH1 and MAML1, in a MAML1-dependent manner (By similarity). Nuclear, in a punctate manner. Widely expressed with highest levels in heart, pancreas, peripheral blood leukocytes and spleen. The C-terminal region is required for transcriptional activation. Belongs to the mastermind family.