TTN and hydrops fetalis: Despite AF being traditionally considered an ion channelopathy, the sarcomeric gene, TTN, is the most common in which rare loss-of-function variants have been associated with AF (Choi et al., 2018; Yoneda et al., 2021), and the penetrance of such variants was found to be even greater for AF than for HF in the UK Biobank (Choi et al., 2020).