Hela Kyoto (HK) cells were grown in Dulbecco's modified Eagle's medium (DMEM) (Sigma) supplemented with 10% fetal calf serum (FCS; Sigma) and 1% penicillin/streptomycin at 37°C and 5% CO2. Transfection was performed as described in (19 (link)) using X-tremeGene (Roche Applied Science) according to the manufacturer's instruction. Expression levels of HK cells stably transfected with eGFP and eGFP-tagged histone H3 variants and mutants were quantified using a FACSCanto machine (Becton Dickinson). Human primary mesenchymal stem cells were purified from lipoaspirates and cultured and transfected by electroporation using Nucleofector™ Technology (Lonza) as described in (20 (link)).