Model for the role of Tip60 in DSB repair. Tip60 appears to play a role in DSB repair through its acetyltransferase activity. Tip60 displays HAT (histone acetyltransferase) activity by acetylating histones H2A and H4 (nucleosomes are represented by green barrels) which may have important consequences in response to DSBs. Tip60 has been suggested to play a direct role in the activation ATM (a crucial DSB repair protein kinase) by acetylating it in response to DNA damage. Following the induction of DNA damage, Tip60 has also been shown to associate with ATM. In addition, Tip60 is an indispensable regulator of p53 function and acetylates transcription factors, which may play a role in DNA damage response pathways. Tat may influence all of these events, because it inhibits Tip60 activity and regulates Tip60 degradation.