Promoter histone marks and heterochromatin factor HP1a binding. Chromatin immunoprecipitation (ChIP) assay was performed with anti-trimethyl H3K4 (A), anti-trimethyl H3K9 (B), anti-dimethyl H3K27 (C) and anti-HP1a (D) antibodies. Input DNA of each group was amplified by real-time qPCR as a positive control. Stable copGFP clone cells were transiently transfected with 1 μg blank pcDNA3.1 or suppressor vectors. Forty-eight hours post-transfection, cells were harvested for ChIP assay. a: P <0.05 versus pcDNA3.1 blank vector; b P <0.05 versus KRAB group; and c P <0.05 versus Sss1 group.