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The ability of the humoral immune system to generate Abs capable of specifically binding a myriad of Ags is critically dependent on the somatic hypermutation program. This program induces both templated mutations (ie, gene conversion) and untemplated mutations. In humans, somatic hypermutation is widely believed to result in untemplated point mutations. In this study, we demonstrate detection of large-scale templated events that occur in human memory B cells and circulating plasmablasts. We find that such mutations are templated intrachromosomally from IGHV genes and interchromosomally from IGHV pseudogenes as well as other homologous regions unrelated to IGHV genes. These same donor regions are used in multiple individuals, and they predominantly originate from chromosomes 14, 15, and 16. In addition, we find that exogenous sequences placed at the IgH locus, such as LAIR1, undergo …