Multi-platform framework for mapping somatic retrotransposition in human tissues.

Long-read and MEI-targeted sequencing, integrated with haplotype phasing and donor-specific assemblies, enabled detection of rare somatic L1 insertions in all four examined human tissue types.

• Why it matters: Somatic mobile element insertions are difficult to find in non-cancer tissues because they are ultra-rare and embedded in repetitive sequence, leaving limited ability to map where they occur and which elements generate them.
• What they did: The authors benchmarked sMEI detection methods and built a cross-platform integrative pipeline using mixtures of well-established cell lines, then used phasing and donor-specific genome assemblies to separate germline from false signals in in silico tumor-normal mixtures. They also created a source-tracing method using sequence variation within MEIs to expand active source element identification.
• The result: In benchmark donor tissues, the framework detected rare somatic L1 insertions across all four tissue types and revealed structural and source diversity, supporting a more complete catalog of active retrotransposition sources.

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