Histone H3 is a core histone protein, which complexes with the other histones to form the major constituents of chromatin in eukaryotic cells. In mammalian cells, phosphorylation of Threonine 3 residue in histone H3 reaches a maximum for condensation during mitosis. Phosphorylation of histone H3 (pHH3) occurs only during late G2 phase and mitosis. pHH3 is a marker for mitoses in various types of tumors. It is particularly useful in identifying mitotic figures in tumors with dense cellularity, limited mitotic activity, and/or abundant apoptotic, pyknotic or distorted nuclei.