RAPGEF4 and cardiac arrhythmia: This study aimed to evaluate whether CD44, a transmembrane receptor mediating various cellular responses, participates in Epac-dependent arrhythmias.<h4>Methods</h4>The heart tissue from CD44 knockout (CD44<sup>-/-</sup>) mice, cultured HL-1 myocytes and the tissue of human ventricle were used for western blot, co-immunoprecipitaiton and confocal studies.