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... the differences between cellular replication and viral model systems. The human DNA polymerases ? and ? were found capable of gap-filling DNA synthesis during nucleotide excision repair in vitro. Both enzymes required PCNA and the clamp loader RFC, and in addition, polymerase ? required Fen-1 to prevent excessive displacement synthesis. Nucleotide excision repair of a defined DNA lesion was completely reconstituted utilising largely recombinant proteins, only ligase I and DNA polymerases ? and ? provided as highly purified human enzymes. This system was also utilised to study the role of the transcription factor II H during repair. Human non-homologous end joining of model substrates with different DNA end configurations was studied in HeLa cell extracts. This process depended partially on DNA synthesis as an aphidicolin-dependent DNA polymerase was required for the formation of a subset of end joining products. Experiments with neutralising antibodies reveal that DNA polymerase ? but not ...