Base excision DNA repair (BER) is a vitally important pathway that protects the cell genome from
many kinds of DNA damage, including oxidation, deamination, and hydrolysis. It involves several tightly coordinated
steps, starting from damaged base excision and followed by nicking one DNA strand, incorporating an
undamaged nucleotide, and DNA ligation. Deficiencies in BER are often embryonic lethal or cause morbid diseases
such as cancer, neurodegeneration, or severe immune pathologies. Starting from the early 1980s, when the
first mammalian cell lines lacking BER were produced by spontaneous mutagenesis, such lines have become a
treasure trove of valuable information about the mechanisms of BER, often revealing unexpected connections
with other cellular processes, such as antibody maturation or epigenetic demethylation. In addition, these cell
lines have found an increasing use in genotoxicity testing, where they provide increased sensitivity and representativity
to cell-based assay panels. In this review, we outline current knowledge about BER-deficient cell lines
and their use.