mechanisms of genetic instability caused by cggn we developed an experimental system for studying genome instability caused by fragile x cggn repeats in mammalian cells our method uses a selectable cassette carrying the hytk gene under the control of the fmr1 promoter with cggn repeats in its 5 utr which is integrated into the unique rl5 site in murine erythroid leukemia cells carriersize cggn repeats markedly elevated the frequency of reporter inactivation making cells ganciclovir resistant these resistant clones had a unique mutational signature a change in repeat length concurrent with mutagenesis in the reporter gene inactivation of genes implicated in breakinduced replication including pold3 pold4 rad52 rad51 and smarcal1 reduced the frequency of ganciclovirresistant clones to the baseline level that was observed in the absence of cggn repeats we propose that replication fork collapse at carriersize cggn repeats can trigger breakinduced replication which results in simultaneous repeat length changes and mutagenesis at a distance