Short tandem repeat sequences in the mammalian genome are considered to be unstable, since many of them are polymorphic in length; however, the extent of this instability has been difficult to quantitate. We have directly determined the rate of mutation of a simple sequence repeat in a mammalian cell line. A plasmid containing a dinucleotide repeat [poly(CA/GT)] that disrupts the reading frame of a downstream gene was integrated into the genome of a mouse cell line, and spontaneous revertants were selected. Reversion rates were more than 100 times higher in cells carrying the repeated sequence than in control cells that carried the same fusion gene with a 4-bp out-of-frame deletion. Revertant clones derived from the lines carrying the dinucleotide repeat had insertions or deletions of one or more repeat units.