Phosphohistone H3 (PHH3) is a core histone protein, which together with other histones, forms the major protein constituents of the chromatin in eukaryotic cells. In mammalian cells, phosphohistone H3 is negligible during interphase but reaches a maximum for chromatin condensation during mitosis. Anti-Phosphohistone H3 (PHH3) has been shown to be specific for the core protein histone H3 only when phosphorylated at serine 10 or serine 28, deeming PHH3 useful as a mitotic marker for differentiating mitotic figures from apoptotic bodies and karyorrhectic debris, since phosphorylation on histone H3 is not detected during apoptosis. Phosphohistone H3 may therefore be useful in diagnosis of tumor grades, especially in the central nervous system, skin, gynecological, soft tissue, and gastrointestinal stromal tumors. Immunohistochemical staining with Anti-PHH3 has also been considered highly useful in prognosis of breast cancer, melanoma, and meningiomas.