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The mammalian family X DNA polymerases (DNA polymerases beta, lambda, mu, and TdT)
contribute to base excision repair and double-strand break repair by virtue of their
ability to fill short gaps in DNA. Structural information now exists for all four
of these enzymes, making this the first mammalian polymerase family whose structural
portrait is complete. Here we consider how distinctive structural features of these
enzymes contribute to their biological functions in vivo.