chromatin insulators demarcate expression domains by blocking the cis effects of enhancers or silencers in a position dependent manner we show that the chromatin insulator protein ctcf carries a post translational modification poly adp ribosyl ation chromatin immunoprecipitation analysis showed that a poly adp ribosyl ation mark which exclusively segregates with the maternal allele of the insulator domain in the h19 imprinting control region requires the bases that are essential for interaction with ctcf chromatin immunoprecipitation on chip analysis documented that the link between ctcf and poly adp ribosyl ation extended to more than 140 mouse ctcf target sites an insulator trap assay showed that the insulator function of most of these ctcf target sites is sensitive to 3 aminobenzamide an inhibitor of poly adp ribose polymerase activity we suggest that poly adp ribosyl ation imparts chromatin insulator properties to ctcf at both imprinted and nonimprinted loci which has implications for the regulation of expression domains and their demise in pathological lesions.