DNA damage activates signaling pathways that lead to modification of local 
chromatin and recruitment of DNA repair proteins. Multiple DNA repair proteins 
having ubiquitin ligase activity are recruited to sites of DNA damage, where 
they ubiquitinate histones and other substrates. This DNA damage-induced histone 
ubiquitination is thought to play a critical role in mediating the DNA damage 
response. We now report that the polycomb protein BMI1 is rapidly recruited to 
sites of DNA damage, where it persists for more than 8 h. The sustained 
localization of BMI1 to damage sites is dependent on intact ATM and ATR and 
requires H2AX phosphorylation and recruitment of RNF8. BMI1 is required for DNA 
damage-induced ubiquitination of histone H2A at lysine 119. Loss of BMI1 leads 
to impaired repair of DNA double-strand breaks by homologous recombination and 
the accumulation of cells in G(2)/M. These data support a crucial role for BMI1 
in the cellular response to DNA damage.