Chi sites serve as stimulators of DNA double-strand break repair in bacteria, which can arise from radiation or chemical treatments, or result from replication fork breakage during DNA replication.
The Chi sequence serves as a signal to the RecBCD helicase-nuclease that triggers a major change in the activities of this enzyme.
In either case the resulting 3’ single-stranded DNA (ssDNA) is bound by multiple molecules of RecA protein that facilitate "strand invasion," in which one strand of a homologous double-stranded DNA is displaced by the RecA-associated ssDNA.
Strand invasion forms a joint DNA molecule called a D-loop.
In reference to E. coli phage lambda, the term is sometimes written as "χ site", using the Greek letter chi; for E. coli and other bacteria the term "Chi" is proper.