Component of heterochromatin that maintains heterochromatin integrity during G1/S progression and regulates the duration of G1 phase to critically influence cell proliferative capacity. May play a role in hypoxia-induced oncogenesis. Interacts (via PxVxL motif) with CBX5 (via Trp-174). localized in nuclei but not in nucleoli in interphase. Colocalized with chromosomes in mitosis, with a gradually increased during G1 progression and a maximum level during late G1 phase (G1/S). A central region that included the first H15 (linker histone H1/H5 globular) domain binds at the entry/exit site of the nucleosomal DNA.