Science1 publisherNot yet confirmed elsewhere2 min readPublished
Cell mutations show one sister's cancer crossed to her twin late in the second trimester
Researchers used identical twin girls' own cell mutations to show one sister's cancer crossed to the other before birth, in a single late transfer. The lineage method, reported in Nature Communications, also dated the tumor's origin to the end of the first trimester and traced how the sisters formed.
The Scientist · Science desk
What happened
- A fetal MRI at 33 weeks found the mass on one twin's face, and both sisters were born with tumors about a week later.
- The twins, Amelia and Amaya, died soon after birth, and their family consented to a research post-mortem.
- The tumors were undifferentiated sarcomas, and every sample carried the same fusion of the MN1 and ZNF341 genes.
- Twin B's tumors were built from cells carrying twin A's genetic markers, the sign the cancer had begun in twin A.
- Normal cells had also moved the other way, with about 75 percent of a spleen sample from twin A originating in twin B.
Compiled by The ScientistSomething wrong?How this is made
Why it matters
- capability Because the cells keep a record of their own divisions, the method can date disease events from before birth that nobody observed directly.
- constraint The transfer dates come from assuming mutations build up at a constant rate and counting them, so they are estimates tied to that assumption, not measured events.
- precedent This is one reconstruction of a single twin pair, so it cannot say how often cancer crosses between twins, or that a tumor in one must reach the other.
The same cancer in two babies could mean the disease arose twice or crossed once, and the sisters' shared origin alone could not separate those cases [25]. The way in came from a fact that looks like a contradiction: identical twins are not necessarily genetically identical [11]. They begin from one fertilized egg, but DNA changes arise as cells divide, and a change picked up by a single cell is inherited by all of its descendants, marking one branch of the developing body's cellular family tree [11]. Compare those marks across tissues and you can read off shared ancestors and tell one sister's cells from the other's [12].
The team, from the Wellcome Sanger Institute, Great Ormond Street Hospital and the University of Cambridge, worked from 10 tumor samples, 12 normal tissue samples from the twins and 11 placental samples, plus 12 more placental samples enriched for the cells that build the placenta [8][13]. The most likely starting point was twin A's facial mass, where the cancer diversified and spread through her body before reaching her sister across the placenta they shared [16][15]. The genetics could name the source twin, but not show the cells in transit or the route they took [15].
The twins' earliest cell branches had fed their bodies and the placenta unevenly. Nearly all of one branch ended up in twin A. A second branch fed both sisters as well as the placenta, and a third fed the placenta and twin B [20]. That spread favored an earlier embryonic split than the classical model for this kind of identical twinning predicts [20].
What to watch
- Whether other twin pairs show the same cross-placental cancer transfer, which would start to establish how often it happens.
- Whether the timing holds up when the mutation rate is measured in these tissues rather than assumed constant.
- Whether the earlier embryonic split seen in this pair is confirmed against the classical twinning model in further cases.
Clarity's read
What the record supports and how the coverage leans. The claims behind it follow.
Reality
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- Adoption
- Insufficient
- Hype gap+10
- Incentives
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- [2]
When the identical twin sisters were born about a week after the 33-week scan, tumors were found in both of them.
- [3]
A study in Nature Communications concludes the cancer began in one sister and passed to the other in the womb, most likely through a single transfer late in the second trimester.
- [4]
The same investigation uncovered clues to how the sisters themselves had formed.
- [7]
The authors dedicate the study to the sisters, whose mother wanted them acknowledged by name.
- [8]
The team included researchers from the Wellcome Sanger Institute, Great Ormond Street Hospital and the University of Cambridge.
- [9]
The tumors were reported as undifferentiated sarcomas; soft tissue sarcomas arise in tissues such as muscle, fat and the body's supporting structures.
- [10]
All the twins' tumor samples carried the same fusion of two genes, MN1 and ZNF341.
- [11]
Identical twins begin from the same fertilized egg, but DNA changes can arise as their cells multiply; a change acquired by one cell passes to its descendants, marking a branch of the developing body's cellular family tree.
- [12]
Comparing those marks across tissues reveals shared ancestors and distinguishes one twin's cells from her sister's.
- [13]
The team sequenced 10 tumor samples, 12 normal tissue samples from the twins and 11 placental samples, and examined 12 placental samples enriched for cells involved in forming the placenta.
- [14]
The researchers found cancer began in twin A, then cancer cells passed to twin B before birth and formed tumors there; twin B's tumor cells carried genetic markers belonging to her sister.
- [15]
The study points to spread across the placenta the twins shared; the genetic analysis establishes which sister the cancer came from but does not directly show the cells crossing or the route they took.
- [16]
The most likely starting site was twin A's facial mass; the cancer diversified and spread within her body before reaching her sister, and the pattern in twin B supported a relatively late, single transfer.
- [17]
Assuming mutations built up at a constant rate, the researchers placed the tumor's origin near the end of the first trimester and its transfer late in the second trimester.
- [18]
Those dates are estimates that depend on the assumed mutation rate, rather than direct observations.
- [19]
Genetic markers in a spleen sample from twin A suggested around 75 percent of the sampled cells originated in twin B, consistent with blood-cell mixing between the sisters.
- [20]
The twins' earliest cell lineages contributed unevenly: one branch almost exclusively to twin A, another to both sisters and the placenta, and a third to twin B and the placenta; the pattern favored an earlier embryonic split than the classical model for this type of identical twinning predicts.
- [21]
This was one pair of twins, and the analysis could recover only lineages that survived in the tissues sampled.
- [22]
Earlier research on twins with leukemia has also explored cancer's beginnings before birth; this case adds a detailed reconstruction of a soft tissue tumor's journey, without establishing how often such transfers occur.
- [23]
The study does not show that a cancer in one twin must reach the other.
- [24]
DNA changes accumulated as their bodies developed allowed the team to reconstruct events that had happened months before anyone saw a tumor.
- [25]
Two children with the same cancer raised a question their shared beginnings alone could not answer: whether the disease started separately in each sister or crossed from one to the other before birth.
Sources
1 independent publisher whose own reporting we read for this story.
- sciencealert.comScientists Trace How Cancer Passed Between Twins Before Birth
1 article · October 8, 2026
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