Sorted CD4+ CD8+ DP thymocytes were fixed in 10% FCS DMEM media containing 1% formaldehyde. The cells were lysed and sonicated in [10 mM Tris-HCl, pH7.4, 800 mM NaCl, 1mM EDTA, 0.1% SDS, 1% Triton X, 0.1% Sodium Deoxycholate, protease inhibitors]. Lysates were cleared by centrifugation and 150 μg were incubated with 5 μg anti-Rad21 antibody (Abcam, ab992) and 25 μl Dynabeads to capture Rad21-DNA complexes. DNA was eluted with a mixture of RNaseA and proteinaseK for 2 hours at 37° followed by 16 hours at 65° to reverse the cross-links. DNA was purified by phenol-chloroform extraction and 10ng was used with a Illumina ChIP-seq sample preparation according to the manufacturer instructions.