Eukaryotic cells must possess mechanisms for condensing and decondensing chromatin. Chromatin condensation is particularly evident during mitosis and cell death induced by apoptosis, whereas chromatin decondensation is necessary for replication, repair, recombination and transcription. Histones are among the numerous DNA-binding proteins that control the level of DNA condensation, and post-translational modification of histone tails plays a critical role in the dynamic condensation/decondensation that occurs during the cell cycle. Phosphorylation of Ser10 in the tails of histone H3 has been extensively studied in many organisms. Interestingly, this modification is involved in both transcription and cell division, two events requiring opposite alterations in the degree of chromatin compaction. How does one and the same modification of histone H3 fulfil such roles? For instance, in interphase, phosphorylation of H3 correlates with chromatin relaxation and gene expression, whereas in mitosis it correlates with chromosome condensation. What is the kinase and under what circumstances does Ser10 becomes phosphorylated? Most importantly, what are the consequences of phosphorylation of this residue?