POLH and Xeroderma pigmentosum variant: To further challenge the assay, we used fibroblasts from patients with xeroderma pigmentosum variant (XP‐V) lacking functional DNA polymerase eta (Polη, encoded by the gene POLH), a key enzyme involved in translesion synthesis (TLS) and DDT (Menck & Munford, 2014), as well as POLH‐complemented counterparts.