In cultures of ventricular cardiomyocytes, expression of LQTS-associated CaM mutations leads to prolonged action potentials (APs), in some cases spilling over to the next stimulation and causing alternans (Limpitikul et al., 2014, 2017; Gomez-Hurtado et al., 2016; Yamamoto et al., 2017). The gene discussed is CALM1; the disease is familial long QT syndrome.