Antibodies that penetrate cell nuclei and inhibit DNA repair or interfere with DNA metabolism are provided for treatment of cancer (both directly and by sensitizing cancer cells to DNA-damaging treatments) or inhibiting or preventing viral infection, proliferation or metabolism. The method involves treating cells with a composition containing cell-penetrating anti-DNA antibodies or derivatives thereof, alone or in combination with treatment that induces DNA damage such as DNA-damaging chemotherapy or radiation. The impact of the cell-penetrating anti-DNA antibodies or derivatives thereof is potentiated in cancer cells that are deficient in DNA repair, and the cell-penetrating anti-DNA antibodies or derivatives thereof are synthetically lethal to cancer cells with DNA repair deficiencies.