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 (PubMed:25038227). Nuclear, in a punctate manner. At E9.5, strongly expressed in the telencephalon, first branchial arch, forelimb buds and somites. By 10.5 dpc, continuously expressed in brain and spinal cord. Also expressed in first and second branchial arches and limb buds. By 11.5 dpc, expression in CNS is weak but increases in mesodermal tissues. At 14.5 dpc, detected in epithelial cells in trachea, esophagus and proximal and distal tubules of the developing lungs. The C-terminal region is required for transcriptional activation. Belongs to the mastermind family.