The nucleotide-replacement spectrum under somatic hypermutation exhibits microsequence dependence that is strand-symmetric and distinct from that under germline mutation.

Somatic mutation is a fundamental component of acquired immunity. Although its molecular basis remains undetermined, the sequence specificity with which mutations are introduced has provided clues to the mechanism. We have analyzed data representing over 1700 unselected mutations in V-gene introns and non-productively rearranged V genes to identify the sequence specificity of the mutation spectrum---the distribution of resultant mucleotides. In other words, we sought to determine what effects the neighboring bases have on what a given base mutates to. We find that both neighboring bases have a significant effect on the mutation spectrum. Their influences are complicated but much of the effect can be characterized as enhancing homogeneity of the mutated DNA sequence. In contrast to what has been reported for the sequence specificity of the ``targeting'' mechanism, that of the spectrum is notably symmetric under complementation, indicating little if any strand bias. We compared the spectrum to that found previously for germline mutations as revealed by analyzing pseudogene sequences. We find that the influences of nearest neighbors are quite different in the two datasets. Altogether, our findings suggest that the mechanism of somatic hypermutation is complex, involving two or more stages: introduction of mis-pairs and their subsequent resolution, each with distinct sequence specificity and strand bias.