Given the critical nature of myosin isoforms regarding contractile function, we further validated this by gel electrophoresis (Fig. S1), and with a targeted absolute quantification mass spectrometry approach known as Multiple Reaction Monitoring (MRM, assay development shown in Fig. S2) which showed a 50% increase in βMHC expression in the AF atria (Fig. 1D). Here, MYH14 is linked to atrial fibrillation.