The impact of chemical inhibition of Pol ζ activity on overall genome stability should also be considered, since genetic ablation of REV3L encoding the catalytic subunit of Pol ζ increased genome instability in REV3L-null mouse embryo fibroblasts (206), and contributed to development of lymphomas and mammary tumors in mice where REV3L was conditionally deleted (207). Here, REV3L is linked to breast cancer.