There was also a substantial number of loci that were not co-occupied by bio-AT7519 and its target CDK9 which might be attributed to the weaker signal obtained for bio-AT7519 or because AT7519 can bind and inhibit other cyclin-dependent kinase like cdks 1, 2, 4, 5.[1][2] In a third experiment, Chem-seq was efficient in mapping genomic binding sites of the DNA intercalating agent psoralen and showed that bio-psoralen preferentially binds to the transcription start site of active genes.