Completion of lagging strand DNA synthesis requires processing of up to 50 
million Okazaki fragments per cell cycle in mammalian cells. Even in yeast, the 
Okazaki fragment maturation happens approximately a million times during a 
single round of DNA replication. Therefore, efficient processing of Okazaki 
fragments is vital for DNA replication and cell proliferation. During this 
process, primase-synthesized RNA/DNA primers are removed, and Okazaki fragments 
are joined into an intact lagging strand DNA. The processing of RNA/DNA primers 
requires a group of structure-specific nucleases typified by flap endonuclease 1 
(FEN1). Here, we summarize the distinct roles of these nucleases in different 
pathways for removal of RNA/DNA primers. Recent findings reveal that Okazaki 
fragment maturation is highly coordinated. The dynamic interactions of 
polymerase δ, FEN1 and DNA ligase I with proliferating cell nuclear antigen 
allow these enzymes to act sequentially during Okazaki fragment maturation. Such 
protein-protein interactions may be regulated by post-translational 
modifications. We also discuss studies using mutant mouse models that suggest 
two distinct cancer etiological mechanisms arising from defects in different 
steps of Okazaki fragment maturation. Mutations that affect the efficiency of 
RNA primer removal may result in accumulation of unligated nicks and DNA 
double-strand breaks. These DNA strand breaks can cause varying forms of 
chromosome aberrations, contributing to development of cancer that associates 
with aneuploidy and gross chromosomal rearrangement. On the other hand, 
mutations that impair editing out of polymerase α incorporation errors result in 
cancer displaying a strong mutator phenotype.