Initiation factor eIF3 plays a central role in the initiation pathway, influencing ribosome association, ternary complex binding to 40S subunits, and mRNA binding, in part through an interaction with eIF4F. We are attempting to clone and sequence DNAs encoding the subunits of this complex factor. Mammalian eIF3 comprises 10 subunits; full-length human cDNAs have been cloned for eight of these, and partial clones are in hand for the remaining two. Yeast eIF3 comprises at least seven subunits, with six of the seven genes identified and sequenced. Comparison of eIF3 subunit sequences between human and yeast reveals an unexpectedly large diversity of structure. Surprisingly, comparisons with other sequences in the data base suggest that some of the eIF3 subunits may have functions apart from the eIF3 complex. Work is in progress to use the cloned DNAs as tools for elucidating the structure of eIF3 and its interactions with other initiation factors.