Serine/threonine-protein kinase that phosphorylates histone H3 at 'Thr-4' (H3T3ph) during mitosis and interphase (PubMed:32750047). Function is essential for chromosome organization during mitosis and genome organization in interphase cells, thus playing a functional role in gene regulation (PubMed:32750047). During mitosis, may act through H3T3ph to both position and modulate activation of AURKB and other components of the chromosomal passenger complex (CPC) at centromeres to ensure proper chromatid cohesion, metaphase alignment and normal progression through the cell cycle (By similarity). During interphase, associates with the cohesion complex and mediates pds5 binding to chromatin to ensure correct sister chromatid cohesion, chromatin organization, and also functions with Pds5-cohesin to modify Polycomb-dependent homeotic transformations (PubMed:32750047). Function during interphase is required for insulator activity, nuclear compaction, heterochromatin-induced position-effect variegation and PcG-mediated pairing-sensitive silencing (PubMed:32750047). ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Interacts with pds5 and vtd. Viable, however adults display a decrease in survival which is stronger in females than males, and both sexes display a decrease in fertility (PubMed:32750047). Loss of phosphorylation of histone H3 leading to reduced centromeric cohesion (PubMed:32750047). RNAi-mediated knockdown reduces the size of salivary gland nuclei (PubMed:32750047). Also results in a strong decrease in chromosome-bound pds5 in larval salivary glands and a significant decrease in chromatin-bound vtd in mitotic and interphase embryo cells (PubMed:32750047). Belongs to the protein kinase superfamily. Ser/Thr protein kinase family. Haspin subfamily.