# Cancer crossed the placenta between newborn twins > Whole-genome sequencing traced one rare soft-tissue cancer from one newborn twin to the other before birth, while showing why a single case cannot establish how often this happens. _Source: Telegram lead from CNSmydream, checked against the peer-reviewed Nature Communications study and independent coverage by ScienceAlert · 2026-10-28 · 6 min read · Verified against primary sources_ Canonical: https://iyu.app/e/embryonic-cancer-twin-transfer ## The 60-second version Sequencing suggests a soft-tissue cancer began in one newborn twin and crossed the shared placenta once before birth. **Key points** - The twins' tumors shared an MN1–ZNF341 gene fusion and were studied alongside normal and placental tissues. - Genetic lineage tracing placed the origin in the twin with a prenatal facial mass and found her markers in the other twin's tumors. - The inferred transfer timing and route are based on mutation patterns, while the study's single-family design cannot estimate frequency. **Verdict.** The study reveals a rare prenatal cancer-transfer mechanism and a powerful reconstruction method, not a general risk for identical twins. ## Full explainer > **⚑ Caveat:** This is a study of one twin pair. The transfer route and timing were reconstructed from genetic evidence, not observed directly, and the result does not show how often cancer passes between twins. ### The case — One cancer, two newborns Two identical newborn sisters presented with matching undifferentiated sarcomas, a soft-tissue cancer whose cells do not show a clear mature identity. One twin had a facial mass detected before birth and tumors in several organs at delivery. The other had skin lesions and later-detected disease around the membranes of the brain and spinal cord. Both tumors carried the same MN1–ZNF341 gene fusion. That was a clue to a shared origin, but it did not by itself reveal whether the cancers began independently or whether one tumor had reached the other twin. ### The evidence — A genetic family tree points to one starting twin The researchers used whole-genome sequencing on 10 tumor samples, 12 normal-tissue samples from the twins, and 23 placental samples. Mutations that appear as cells divide can act as timestamps and branch marks: they show which cells share an ancestor and which developmental path they followed. - **10** — tumor samples sequenced - **12** — normal-tissue samples from the twins - **23** — placental samples examined The reconstructed lineage placed the tumor's origin in the twin with the facial mass. Tumors in the other twin carried markers that belonged to her sister, supporting a transfer from the first twin rather than two fully separate cancers. ### The route — A single prenatal transfer is the best fit The authors infer that the cancer diversified in the first twin, then crossed the shared placenta once and seeded tumors in the second. Mutation accumulation placed the initial origin near the end of the first trimester and the transfer late in the second trimester. > **↗** The dates are estimates based on an assumed mutation rate. Sequencing can reconstruct the most likely order of events, but it cannot serve as a recording of cells crossing the placenta. ### The developmental clue — Identical does not mean perfectly symmetrical The same samples also revealed uneven contributions from early embryonic cell lineages. One branch contributed mostly to one twin, another to both twins and the placenta, and a third to the second twin and the placenta. The pattern challenges the classical picture of identical twinning as a completely even split. A spleen sample from the first twin also suggested that many sampled blood cells originated from her sister, consistent with blood-cell mixing between the twins. That finding adds context to the cancer story: the shared prenatal environment can contain a complex traffic of cells even when the twins begin from one fertilized egg. ### What follows — A rare mechanism, not a general rule The study examined one family, and both children died soon after birth. It shows that a prenatal transfer can occur in this rare setting; it does not establish a frequency, explain every cancer found in twins, or show that sharing a placenta makes transmission inevitable. > The lasting contribution is a method for reading tumor evolution and early human development from the same cellular record. For researchers, the case offers a way to study when a childhood tumor began, how it diversified, and how it moved. For readers, the practical conclusion is narrower: this is an extraordinary case report, not evidence that identical twins generally pass cancer to each other. ## Primary sources - [Telegram lead, CNSmydream post 1562](https://t.me/CNSmydream/1562) - [Nature Communications original study](https://doi.org/10.1038/s41467-026-78178-5) - [Nature article page and abstract](https://www.nature.com/articles/s41467-026-78178-5) - [ScienceAlert independent report](https://www.sciencealert.com/scientists-trace-how-cancer-passed-between-twins-before-birth) - [PubMed review on gestational and perinatal cancer transmission](https://pubmed.ncbi.nlm.nih.gov/38369705/) --- _Published by iyu (https://iyu.app) — the day's AI news, checked against primary sources and rewritten in plain language. Free to quote with attribution and a link to the canonical URL._