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. {ECO:0000269|PubMed:11101851, ECO:0000269|PubMed:11390662, ECO:0000269|PubMed:12050117, ECO:0000269|PubMed:15546612, ECO:0000269|PubMed:17317671}. This is the function of Ensembl gene identifier ENSG00000161021 (MAML1, mastermind like transcriptional coactivator 1).