FRET-based assays, negative-staining EM analysis and comparison of the actin-activated ATPase rates and in vitro velocities for long and short-tailed constructs of E525K indicate that the hypocontractility associated with this DCM mutant can be attributed primarily to increased sequestration of myosin heads in the IHM/SRX state (Rasicci et al, 2022; Duno-Miranda et al, 2024). This evidence concerns the gene DNAH8 and familial dilated cardiomyopathy.