DNA replication normally occurs with high fidelity, but certain "slippery" regions of DNA with tracts of mono-, di-, and trinucleotide repeats are frequently mutation hot spots. We have developed an in vitro assay to study the mechanism of dinucleotide repeat expansion. The primer-template resembles a base excision repair substrate with a single nucleotide gap centered opposite a tract of nine CA repeats; nonrepeat sequences flank the dinucleotide repeats. DNA polymerases are expected to repair the gap, but further extension is possible if the DNA polymerase can displace the downstream oligonucleotide. We report here that the wild type bacteriophage T4 DNA polymerase carries out gap and strand displacement replication and also catalyzes a dinucleotide expansion reaction. Repeat expansion was not detected for an exonuclease-deficient T4 DNA polymerase or for Escherichia coli DNA polymerase I. The dinucleotide repeat expansion reaction catalyzed by wild type T4 DNA polymerase required a downstream oligonucleotide to "stall" replication and 3 3 5 exonuclease activity to remove the 3-nonrepeat sequence adjacent to the repeat tract in the template strand. These results suggest that dinucleotide repeat expansion may be stimulated in vivo during DNA repair or during processing of Okazaki fragments.DNA polymerases replicate DNA with high fidelity except for certain DNA sequences, the so-called "slippery" DNAs, which are tracts of simple repeat sequences (reviewed in Ref. 1). The lengths of tracts of mono-, di-, and trinucleotide repeats are unstable, which can easily be detected as repeat-length polymorphisms in microsatellite sequences or as mutation hot spots, such as the classical frameshift hot spots identified by Benzer in the bacteriophage T4 rII genes (2), which are tracts of six A nucleotides (3). Hypermutability in repeat tracts can have serious consequences for human health as observed for an inherited mutation in the human APC gene, which converts the wild type sequence AAATAAAA to the A 8 mononucleotide tract (4). The A 8 sequence was found to create a small hypermutable region for gene inactivating mutations that predispose carriers to colorectal cancer (4). Streisinger et al. (5) suggested that frameshifts are produced in repeat sequences by a transient separation of the primer and template strands and then misalignment during reannealing to generate an intermediate in which one or more repeats is unpaired. This "slippage" intermediate is usually repaired by postreplication mismatch repair as revealed by the dramatic increase in repeat instability when this repair pathway is inactivated (6 -8).In vitro assays have been used to measure DNA polymerasecatalyzed "reiterative replication" of repeat sequences (for examples, see Refs. 9 -14). The number of repeats can be amplified several hundred-fold by a variety of DNA polymerases, but most of the synthetic DNA substrates used are composed exclusively of repeat sequences. One objective of our studies was to develop an improved in vitro assay utilizing a DNA su...