HEK 293T cells were transfected with the plasmid encoding Flag-HP1β. Transfected cells were collected, and the total protein lysate was incubated with anti-Flag antibody overnight at 4 ° C under rotation. Subsequently, pure protein histone A/G mixed magnetic beads were added and incubated for 2 h at room temperature. They were washed three times and resuspended in an appropriate amount of 1×loading buffer. Samples were denatured by boiling at 100 ° C and subjected to LC-MS analysis.