CLK4 and heart failure: Unexpectedly, echocardiography revealed rapidly developed heart failure in Clk4-cKO mice, as evidenced by a significant increase in the left ventricular internal diameter, diastole (LVID, d), and a significant decrease in the left ventricular ejection fraction (EF) compared to those of MCM littermate controls (Fig. 2d–f, Supplementary Fig. 4 and Supplementary Table 5).