The mammalian translation initiation factor 3 (eIF3),is a multiprotein complex of ~600 kDa that binds to the 40 S ribosome and promotes the binding of methionyl-tRNAi and mRNA. The EIF3S7(p66) is the major RNA binding subunit in this complex. Human eIF3-p66 shares 64% sequence identity with a hypothetical Caenorhabditis elegans protein,presumably the p66 homolog. Deletion analyses of recombinant derivatives of eIF3-p66 show that the RNA-binding domain lies within an N-terminal 71-amino acid region rich in lysine and arginine.