pold3 is haploinsufficient for dna replication in mice the pold3 gene encodes a subunit of the pol dna polymerase complex pold3 orthologs are not essential in saccharomyces cerevisiae or chicken dt40 cells but the schizosaccharomyces pombe ortholog is essential pold3 also has a specialized role in the repair of broken replication forks suggesting that pold3 activity could be particularly relevant for cancer cells enduring high levels of dna replication stress we report here that pold3 is essential for mouse development and is also required for viability in adult animals strikingly even pold3 mice were born at submendelian ratios and of those born some presented hydrocephaly and had a reduced lifespan in cells pold3 deficiency led to replication stress and cell death which were aggravated by the expression of activated oncogenes finally we show that pold3 deletion destabilizes all members of the pol complex explaining its major role in dna replication and the severe impact of its deficiency