APCDD1 and cardiac arrhythmia: Interestingly, while the knockdown of hts did not cause any significant heart phenotypes (S4A–S4F Fig), the co-expression of htsRNAi and Gcn5RNAi significantly aggravated the heart period length and the arrhythmia index observed upon single knockdown of Gcn5 (S4A and S4C Fig).