Acts as a transcriptional coactivator for NOTCH proteins. Has been shown to amplify NOTCH-induced transcription of HES1. Potentiates activation by NOTCH3 and NOTCH4 more efficiently than MAML1 or MAML3. {ECO:0000269|PubMed:12370315; ECO:0000269|PubMed:12386158; ECO:0000269|PubMed:12539049}.