Constitutive binding of 53BP1 to chromatin is RNF8 independent. RNF8^+/+, RNF8^−/−, and 53BP1^−/− B cells were stimulated with LPS/IL-4 for 3 d and were treated with IR (10 Gy) or left untreated. After 90 min, cells were collected and chromatin and nuclear fractions were isolated (see Materials and methods). Immunoblotting was performed using the indicated antibodies. One out of two independent experiments is shown.