Cells are continually exposed to environmental insults that damage DNA. If this damage escapes detection by DNA repair enzymes it can stall the DNA replication machinery and lead to cell death. To prevent this, cells recruit specialized DNA polymerases that can replicate past the damage. These "lesion bypass" polymerases are often error-prone--they show low accuracy in reading the template DNA and introduce mutations into the newly formed DNA.

Three research groups (Bemark et al., Wittschieben et al., Esposito et al.) independently created mice deficient in one of these error-prone polymerases, pol z. Absence of the enzyme caused death of the mice in utero due to defects in many different embryonic tissues. This absolute requirement for pol z is surprising in light of the large number of lesion bypass polymerases that might be expected to compensate for its loss and because pol z is not required for viability in yeast. Thus, pol z may perform another yet-to-be-discovered function that is crucial in the early stages of mammalian development. -- PAK