Endocardial cells play important roles in heart formation and function but have generally been
considered as a homogenous cell population, i.e., one that does not exhibit differences according to the
chamber they belong to. In a screen for chamber specific transcripts in the adult zebrafish heart, we
identified meis2b, a homeobox transcription factor gene that is surprisingly expressed specifically in the
endocardial cells of the atrium, including those lining the atrial face of the cardiac valve. This proposal is
focused on understanding the role of meis2b during endocardial development and function.