We report the synthesis, characterization, and in vivo application of light-activated, negatively charged peptide nucleic acids (caged ncPNAs) for down-regulating gene expression during zebrafish embryonic development. The 18-mer ncPNAs were attached to a short complementary 2'-OMe RNA sense strand (sRNA) via a heterobifunctional photocleavable linker (PL). Caged ncPNAs were designed to target chordin and bozozok mRNA. Modest UV irradiation cleaved the photoactive linker, which yielded the less stable ncPNA/sRNA duplex (Delta T (m) similar to -20 degrees C for chordin and -41 degrees C for bozozok ). These caged ncPNAs were microinjected into zebrafish (Danio rerio ) embryos at the one- or two-cell stage. The conjugate was nontoxic, and most embryos showed no phenotypic response at 24 h post fertilization (24 hpf), which indicated low background activity. Embryos that were photoactivated at 2 or 3 hpf developed phenotypic abnormalities typical of embryos with loss of function of the chordin or bozozok gene. Phenotypic results were further confirmed by in situ hybridization experiments using molecular markers. Caged ncPNAs provide a promising "on -> off" switch for down-regulating gene expression with high spatiotemporal control in biological systems.