Under conditions of severe DNA damage the nuclear enzyme poly(ADP-ribose) polymerase 1 (PARP-1) is activated, catalyzing the modification of proteins by forming and attaching to them poly(ADP-ribose) chains. A specific physical interaction between PARP-1 and transcription factor Yin Yang 1 (YY1) in vitro was shown previously, which had important consequences for the activities of both proteins. It is demonstrated here that YY1 and PARP-1 form complexes in vivo. YY1 was transiently poly(ADP-ribosyl)ated immediately after genotoxic treatment of HeLa cells. The narrow time frame of the modification coincides with that known for the activation of PARP-1 under these conditions. This immediate modification correlated with a decreased affinity of YY1 to its cognate DNA binding sites.