As CS cells, as well as cells from the mild UVs syndrome, suffer from elevated reactive oxygen species (ROS), presumably a consequence of mitochondrial affection (Alupei et al. 2018; Kamenisch et al. 2010; Okur et al. 2020) we investigated ROS and oxidative damage in the XPG mutant cells. The gene discussed is ERCC5; the disease is Cowden syndrome 1.