Genetically unstable expanded CAG·CTG trinu- cleotide repeats are causal in a number of human dis- orders, including Huntington disease and myotonic dystrophy type 1. It is still widely assumed that DNA polymerase slippage during replication plays an im- portant role in the accumulation of expansions. Nev- ertheless, somatic mosaicism correlates poorly with the proliferative capacity of the tissue and rates of cell turnover, suggesting that expansions can oc- cur in the absence of replication. We monitored CAG·CTG repeat instability in transgenic mouse cells arrested by chemical or genetic manipulation of the cell cycle and generated unequivocal evidence for the continuous accumulation of repeat expansions in non-dividing cells. Importantly, the rates of expan- sion in non-dividing cells were at least as high as those of proliferating cells. These data are consistent with a major role for cell division-independent expan- sion in generating somatic mosaicism in vivo. Al- though expansions can accrue in non-dividing cells, we also show that cell cycle arrest is not sufficient to drive instability, implicating other factors as the key regulators of tissue-specific instability. Our data reveal that de novo expansion events are not lim- ited to S-phase and further support a cell division- independent mutational pathway.