Flap endonuclease 1 (FEN1) has been shown to remove 5' overhanging flap 
intermediates during base excision repair and to process the 5' ends of Okazaki 
fragments during lagging-strand DNA replication in vitro. To assess the in vivo 
role of the mammalian enzyme in repair and replication, we used a gene-targeting 
approach to generate mice lacking a functional Fen1 gene. Heterozygote animals 
appear normal, whereas complete depletion of FEN1 causes early embryonic 
lethality. Fen1(-/-) blastocysts fail to form inner cell mass during cellular 
outgrowth, and a complete inactivation of DNA synthesis in giant cells of 
blastocyst outgrowth was observed. Exposure of Fen1(-/-) blastocysts to gamma 
radiation caused extensive apoptosis, implying an essential role for FEN1 in the 
repair of radiation-induced DNA damage in vivo. Our data thus provide in vivo 
evidence for an essential function of FEN1 in DNA repair, as well as in DNA 
replication.