Knockdown of zebrafish tbx5 or mef2c disrupts looping of the heart tube. (A, B) Wild-type zebrafish showing a tightly looped heart tube and development of the pectoral fins. Zebrafish injected with ∼5 ng Tbx5 morpholino show pericardial edema and an unlooped heart tube (C) and no pectoral fin development (arrows indicate absence of fins) (D). (E) Zebrafish treated with ∼5 ng Mef2c morpholino also have an unlooped heart tube, but pectoral fin development is unaffected. (F) Embryo following double knockdown with ∼1.86 ng of tbx5 and mef2c morpholinos, showing pericardial edema and an unlooped heart tube. a, atrium; v, ventricle; f, fin.