Hundreds of microRNAs (miRNAs) are thought to repress thousands of target messenger RNAs (mRNAs), but it has been difficult to study the interaction between individual miRNA-mRNA pairs. Choi et al. developed a way to block the interaction between a miRNA and specific target sites. Antisense morpholino oligonucleotides complementary to miRNA binding sites could protect specific mRNAs from miRNA-mediated repression. Applying this technology during zebrafish embryogenesis, they found that the miRNA miR-430 inhibits an antagonist and an agonist of the Nodal signaling pathway. Loss of repression led to an imbalance of signaling and abnormal development.