The immunoglobulin loci are uniquely unstable regions of the genome which undergo as much mutation and selection in a matter of days as a species can undergo in generations of evolution. We have studied the mutational pattern and targeting of this unusual hypermutation process over the past 16 years. The pattern of somatic mutations in rearranged variable (V) genes differs from the pattern of meiotic mutations, indicating that a different mechanism generates hypermutation than generates spontaneous mutation. Hypermutations begin on the 5' end of rearranged V genes downstream of the transcription initiation site and continue through the V exon and into the 3'-flanking region before tapering off. Mutations are located randomly throughout the DNA sequence and exhibit strand bias. The targeting of mutations to the region in and around the rearranged V gene appears to require interactions between the promoter and downstream intronic DNA sequences. The same mechanism that initiates hypermutation around V genes may also produce double-strand breaks that catalyze homologous recombination between rearranged V genes on two chromosomal alleles. With this data we have built a model of hypermutation which predicts that V-region DNA is destabilized at the nuclear matrix during transcription and undergoes strand breaks.