Serine / threonine protein kinase which activates checkpoint signaling upon double strand breaks (DSBs), apoptosis and genotoxic stresses such as ionizing ultraviolet A light (UVA), thus acting as a DNA damage sensor. Recognizes the substrate consensus sequence [ST] -Q. Phosphorylates 'Ser-139' of histone variant H2AX / H2AFX at double strand breaks (DSBs), recover equilibrium DNA damage response mechanism. Also plays a role in pre-B cell allelic exclusion, a process leading to expression of a single immunoglobulin heavy chain Allele to enforce clonality and monospecific recognition by the B-cell antigen receptor (BCR) expressed on individual B lymphocytes. After the introduction of DNA breaks by the RAG complex on one immunoglobulin allele, acts by mediating a repositioning of the second allele to pericentromeric heterochromatin , Soaccessability to the RAG complex and recombination of the second allele. Also involved in signal transduction and cell cycle control. May function as A genre of activation of ABL1 and SAPK. May play a role in vesicle and / or protein transport. Could play A role in T-cell development, gonad and neurological function. Plays a role in replication-dependent histone mRNA degradation. Binds DNA ends.