Classical non-homologous end joining (c-NHEJ) in humans requires DNA ligase IV (LIG4) to seal DNA double-strand breaks.
NOT BSTOTAL BS
NOT BS — Verdict: True
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The Short Version
The evidence strongly supports this statement. In humans, canonical/classical NHEJ uses the XRCC4–DNA ligase IV complex to complete ligation of DNA double-strand breaks, and major reviews describe LIG4 as required and specific to c-NHEJ. Reports of other ligases acting in end joining refer to alternative, non-classical pathways rather than c-NHEJ.
Caveats
Do not generalize this claim to all end-joining repair: alternative end joining and microhomology-mediated end joining can use other ligases.
LIG4 acts in c-NHEJ as part of a complex, especially with XRCC4; the pathway also depends on additional c-NHEJ factors beyond LIG4 alone.
'Requires LIG4' applies to the canonical human pathway as defined in the cited literature, not to backup repair mechanisms activated when c-NHEJ is compromised.