Serine/threonine-protein kinase which recognizes the substrate consensus sequence [ST]-Q. Required to suppress spontaneous apoptosis of proliferating cells during development, and for their proper differentiation. Required for female fertility. Protects telomeres from fusion, maybe by recruiting or maintaining chromatin-modifying complexes such as Su(var)205/HP1. May activate checkpoint signaling in response to DNA double-stranded breaks induced by low-dose ionizing radiation. May phosphorylate histone H2AV. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Expressed at low levels throughout development and at much higher levels in adult females compared to males. Flies die shortly before or after eclosion with a rough eye phenotype, misshapen wings, and missing or abnormal bristles (PubMed:15256487). Shows increased numbers of spontaneous tumors (PubMed:15256487). Simultaneous knockout of tefu and Clbn/NEMF increases the formation of spontaneous tumors (PubMed:22964637). Belongs to the PI3/PI4-kinase family. ATM subfamily.