AND-1/Ctf4 bridges the CMG helicase and DNA polymerase alpha, facilitating 
replication. Using an inducible degron system in avian cells, we find that AND-1 
depletion is incompatible with proliferation, owing to cells accumulating in G2 
with activated DNA damage checkpoint. Replication without AND-1 causes fork 
speed slow-down and accumulation of long single-stranded DNA (ssDNA) gaps at the 
replication fork junction, with these regions being converted to DNA double 
strand breaks (DSBs) in G2. Strikingly, resected forks and DNA damage 
accumulation in G2, but not fork slow-down, are reverted by treatment with 
mirin, an MRE11 nuclease inhibitor. Domain analysis of AND-1 further revealed 
that the HMG box is important for fast replication but not for proliferation, 
whereas conversely, the WD40 domain prevents fork resection and subsequent 
DSB-associated lethality. Thus, our findings uncover a fork protection function 
of AND-1/Ctf4 manifested via the WD40 domain that is essential for proliferation 
and averts genome instability.