A method of causing cardiomyocyte growth and/or differentiation, the
method comprising exposing a cardiomyocyte to neuregulin (NRG) thereby
activating the MAP kinase pathway in the cardiomyocyte and causing growth
and/or differentiation of the cardiomyocyte. Use of neuregulin,
neuregulin polypeptide, neuregulin derivatives, or compounds which mimic
the activities of neuregulins in the treatment or management of heart
disease and heart failure in a mammal.