The repair of chromosomal double-strand breaks. Accurate repair of a DSB requires an undamaged source of duplicate genetic information—that is, homologous double-stranded DNA. The broken ends are first processed to generate 3′ single-stranded extensions, and both ends are used for strand invasion of the homologous double-stranded DNA to form D-loops. The invading 3′ ends are extended by DNA polymerases.