To ensure genome stability, mammalian cells employ several
DNA
repair pathways. Nonhomologous
DNA
end joining (
NHEJ
) is the
DNA
repair process that fixes double‐strand breaks throughout the cell cycle.
NHEJ
is involved in the development of B and T lymphocytes through its function in V(D)J recombination and class switch recombination (
CSR
).
NHEJ
consists of several core and accessory factors, including Ku70, Ku80,
XRCC
4,
DNA
ligase 4,
DNA
‐
PK
cs, Artemis, and
XLF
. Paralog of
XRCC
4 and
XLF
(
PAXX
) is the recently described accessory
NHEJ
factor that structurally resembles
XRCC
4 and
XLF
and interacts with Ku70/Ku80. To determine the physiological role of
PAXX
in mammalian cells, we purchased and characterized a set of custom‐generated and commercially available
NHEJ
‐deficient human haploid
HAP
1 cells,
PAXX
Δ
,
XRCC
4
Δ
, and
XLF
Δ
. In our studies,
HAP
1
PAXX
Δ
cells demonstrated modest sensitivity to
DNA
damage, which was comparable to wild‐type controls. By contrast,
XRCC
4
Δ
and
XLF
Δ
HAP
1 cells possessed significant
DNA
repair defects measured as sensitivity to double‐strand break inducing agents and chromosomal breaks. To investigate the role of
PAXX
in
CSR
, we generated and characterized
Paxx
−/−
and
Aid
−/−
murine lymphoid
CH
12F3 cells.
CSR
to IgA was nearly at wild‐type levels in the
Paxx
−/−
cells and completely ablated in the absence of activation‐induced cytidine deaminase (
AID
). In addition,
Paxx
−/−
CH
12F3 cells were hypersensitive to zeocin when compared to wild‐type controls. We concluded that
Paxx
‐deficient mammalian cells maintain robust
NHEJ
and
CSR
.