Methods and compositions are provided for the diagnosis and treatment of heart diseases relating to cardiac hypertrophy, and for the regulation of proliferation and differentiation of cardiomyocyte progenitors in vitro. The detection of expression of components of the BAF complex, including, without limitation, detection of expression of Brg1, provides useful methods for early detection, diagnosis, staging, and monitoring of conditions leading to hypertrophy and enlargement of the heart. Manipulation of Brg1 activity provides for therapeutic intervention in the development of cardiac hypertrophy, where methods of decreasing Brg1 activity, e.g. through inhibition of binding, decreasing expression, and the like, reduces cardiac hypertrophy.