GATA5 and 6 are essential but GATA4 is redundant in zebrafish myocardium. Zebrafish embryos were injected with GATA4, 5 and/or 6 morpholinos individually or in combination and analysed by whole mount in situ hybridisation at 26 hpf for the expression of Nkx2.5 (A, B), vmhc (C, D) and cmlc2 (E, F). Anterior views: red brackets and arrowheads identify the cardiac expression. Embryos were classified into unaffected (type +), downregulated (type -) and absent (type --) for expression of nkx2.5 (B), vmhc (D) and cmlc2 (F) and graphically represented. The effect on all three cardiac genes studied was similar for each injection. Depletion of GATA4 alone had little impact whereas loss of GATA5 or 6 caused a reduction in the expression of all three. Combinatorial ablation of GATA4 + 5 and GATA4 + 6 caused a further reduction in expression compared to the ablations of GATA5 or GATA6 alone thus demonstrating functional redundancy for GATA4 in the expression of these cardiac genes. GATA5 + 6 ablation resulted in complete loss of expression demonstrating an additive effect for these two GATAs.