Any of a group of proteins that are required for initiating the translation stage of protein synthesis. Each ribosome is assembled on the messenger RNA (mRNA) chain in two subunits, one smaller than the other. IFs catalyse the binding of an initiator tRNA molecule, which recognizes the start codon of the mRNA, to the smaller subunit of the ribosome. This subunit then binds to the mRNA and the IFs dissociate, allowing the larger ribosomal subunit to bind to the smaller one. IFs also control the rate of translation in some cells.