A, Translation blocking. In the absence of a morpholino (left), the small ribosomal subunit scans the 5′ untranslated region. The large ribosomal subunit is recruited at the translation initiation codon (AUG), to reconstitute a functional ribosome that translates the mRNA. If a morpholino hybridizes in the 5′ untranslated region, or in a region flanking the initiation codon (right), scanning by the small ribosomal subunit is impaired. Consequently, the mRNA is not translated. B, Splice inhibiting. A morpholino hybridizes on the pre-mRNA at an intron-exon junction (right). Consequently, this junction is no longer used and the corresponding exon (orange) is skipped. C, Target protector. The mRNA binds a regulatory molecule (protein or miRNA) that controls its fate (left). If a morpholino hybridizes to the mRNA sequence element normally bound by that molecule, binding is impaired (right). This design allows the developmental functions of regulatory interactions to be addressed.