Linker-insertion mutations were introduced into the cloned adenovirus DNA polymerase gene and the functional effects on the initiation and elongation of DNA in vitro were measured. Essential regions of the polymerase appear to be scattered in patches across the entire molecule and are not limited to the five regions of homology shared with a variety of other replicating polymerases. Thus, the adenovirus DNA polymerase presumably contains active sites that must be formed by distant interactions across the polymerase molecule.