Samples were cross-linked with 1% formaldehyde, immunoprecipitated with immune sera raised against human SPT20 and human ZZZ3 as described (Krebs et al Mol Cell 2011; 44: 410) and captured in Invitrogen protein A/G magnetic beads. After reverse cross-link, pulled-down DNA was purified using a Zymo Research kit, in parallel with 2.5% input chromatin DNA, and used for library preparation. NextFlex ChIP-seq library preparation kit - standard protocol