Enables transcription coactivator activity. Involved in atrioventricular node cell development and positive regulation of myotube differentiation. Acts upstream of or within myoblast differentiation and positive regulation of transcription of Notch receptor target. Located in nucleus. Is expressed in several structures, including branchial arch; central nervous system; genitourinary system; limb bud; and respiratory system epithelium. Orthologous to human MAML1 (mastermind like transcriptional coactivator 1). [provided by Alliance of Genome Resources, Apr 2022]