Chromatin immunoprecipitation was performed using the SimpleChIP® Enzymatic Chromatin IP Kit (Cell Signaling Technology, 9003) following the instructions of the manufacturer. 20 μg of chromatin was incubated overnight with 5 μl H2Aub antibody (Cell Signaling Technology, 8240). 100 ng fly chromatin and 0.5 μg H2Av antibody (ActiveMotif, 61686) was added in each tube as spike-in control. Purified ChIP-DNA was verified by agarose gel to make sure properly fragmented and sent to Sequencing and Microarray Facility at MD Anderson Cancer Center for ChIP-seq. ChIP–seq samples were sequenced using the Illumina HiSeq 2000 .