The control of mRNA translation plays an important role in regulating gene expression in diverse situations. Nutrients, especially amino acids, regulate translation to control the expression of specific proteins including transcriptional activators and ribosomal proteins. Sequence elements in the 5'- or 3'-untranslated regions of mRNAs can control their translation. Hormones such as insulin activate protein synthesis by switching on translation factors required for overall mRNA translation. Viruses employ a range of stratagems to ensure efficient translation of their mRNAs and in some cases to inhibit host cell translation. During development, control of mRNA translation regulates gene expression both spatially and temporally.