Sequential ChIP of PRC1 orthologs at the CDKN2A locus. (A) Sequential ChIP was performed with antibodies against CBX8 followed by CBX6 and CBX7, or CBX6 followed by CBX7 and CBX8 as indicated. An irrelevant antibody was used as a control (shown in grey). The recovered chromatin was subjected to quantitative PCR with primers that detect specific PRC1 enrichment at CDKN2A (as described in Figure 1). (B and C) Equivalent analyses with antibodies against HPH1, HPH2 and HPH3 (B) or RING1 and RING2 (C). ChIP, chromatin immunoprecipitation.