HCN4 and Arrhythmia: MiR-1 is the most abundant microRNA in the adult mouse heart, accounting for 35–40% of the total amount of miRNA (Rao et al., 2009) MiR-1 directly targets connexin-43 (Yang et al., 2007), KcnJ2 (Yang et al., 2007), Kcne1 (Jia et al., 2013), Ncx1 (Kumarswamy et al., 2012), PP2A/RyR2 (Terentyev et al., 2009), and Hcn4 (D’Souza et al., 2014) in the adult heart (reviewed in Liao et al., 2016), contributing to the arrhythmias that we and others observed in miR-1 TG adult hearts.