A) Antisense Arg-MO specifically blocked translation of Arg from the RNA that contained its target sequence, but had no effect on protein translation from a RNA containing a modified 5′-UTR [Arg(UTR*)]. 1ng RNA and 20ng Arg-MO were used. B) Arg-MO (40ng) reduced endogenous Arg protein to about 35% of its original level. C) Expression of Arg-MO (40ng) in early frog embryos resulted in delayed blastopore closure and neurulation, severe reduction of head structures, shortened and bent body axis, and open back phenotypes. D, E) The defects induced by Arg-MO (40ng) were partially rescued by co-expression with the modified Arg(UTR*) RNA (5–10pg).