Notch receptors are involved in cell-fate determination in organisms as diverse as flies, frogs, and humans (1). The 'mastermind' gene has been identified in multiple genetic screens for modifiers of Notch mutations in Drosophila melanogaster (2). In Drosophila, loss-of-function mutations of Notch produce a 'neurogenic' phenotype in which cells destined to become epidermis switch fate and differentiate to neural cells (2). The human homolog, mastermind-like 1 (Mam1), localizes to nuclear bodies (2-4). Mam1 binds to the ankyrin repeat domain of all four mammalian Notch receptors, forms a DNA-binding complex with ICN and RBP-Jk, and amplifies Notch-induced transcription of Hes1 (2). Mam1 is an essential component of the transcriptional apparatus of Notch signaling (5). The gene which encodes Mam1 maps to human chromosome 5 (4).