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Fig. 4 tbx5a mutants and tbx5b morphants display cardiac defects at 48 hours postfertilization (hpf). At 48 hpf, the looping angles (defined as the angle created between the plane of the cardiac atrioventricular junction [AVJ] and the embryo anteroposterior [A/P] axis) were determined for tbx5a mutants and tbx5b morphants. A: Representative green fluorescent protein (GFP) -expressing hearts imaged in the Tg(myl7:EGFP-HsHRAS) transgenic line (ventral views). B: Diagram of the looping angle as measured for the hearts in (A). C: The average looping angle determined from at least 14 embryos per genotype. D: Heart rates were measured at early chamber morphogenesis stage (42 hpf), chosen as the optimal time for balancing maximal cardiac function before end-stage degeneration. The average heart rate in beats per minute (BPM) for 42 hpf embryos is presented (at least 11 embryos per genotype). Relative to controls, the mutants and morphants exhibited a statistical difference of **P < 0.001, or *P < 0.01. (a= anterior, defined here as 0°, p = posterior of the embryo).