- Incorporation of dUMP into DNA is a major source of spontaneous DNA damage, while excision of uracil is not required for cytotoxicity of fluoropyrimidines in mouse embryonic fibroblasts. Andersen, S., Heine, T., Sneve, R., König, I., Krokan, H.E., Epe, B., Nilsen, H. Carcinogenesis (2005)