The HMGN family comprises nuclear proteins that bind to nucleosomes and alter the structure of chromatin. Here, we report that DT40 chicken cells lacking eitherHMGN2orHMGN1a, or lacking both HMGN1aand HMGN2, are hypersensitive to killing by UV irradiation. Loss of both HMGN1aand HMGN2or only HMGN2increases the extent of UV-induced G2 –M checkpoint arrest and the rate of apoptosis.