The set of catalytic proteins required, in addition to mRNA and ribosomes, for protein synthesis to begin. In bacteria three distinct proteins have been identified: IF 1 (8 kD), IF 2 (75 kD) and IF 3 (30 kD). At least 6-8 proteins have been identified in eukaryotes. IFs 1 and 2 enhance the binding of initiator tRNA to the initiation complex.