Global run-on sequencing (GRO-Seq) is a method to map the binding sites of transcriptionally active RNA polymerase II. It involves allowing RNA polymerase II to actively transcribe in the presence of labeled nucleotides, followed by purification and sequencing of the newly synthesized RNA. This provides sequences of RNAs that are currently being transcribed, without prior knowledge of transcription sites. While it directly determines relative transcriptional activity, GRO-Seq is limited to cell cultures and may introduce artifacts during nuclear preparation or transcription run-on.