A Cancer That Crossed From One Twin to Her Sister in the Womb

Jeff's Daily Pick. Two identical sisters were born with the same rare cancer. Their DNA shows it started in one twin and crossed to the other through the placenta.

In the United Kingdom, researchers at the Wellcome Sanger Institute, Great Ormond Street Hospital and the University of Cambridge reconstructed how a rare cancer reached two identical twin girls before birth. The study, published on October 6, 2026 in Nature Communications, shows the tumor began in one sister and spread to the other through the placenta they shared. Both children died shortly after birth, and their mother asked that the work be dedicated to them.

The case

A fetal MRI at 33 weeks showed a mass on the face of one twin, which prompted early delivery a week later. At birth, tumors were found in both girls. The cancer was an undifferentiated sarcoma, a soft tissue tumor driven by a rare gene fusion, MN1::ZNF341. The question was whether it had arisen twice, from something both twins shared, or once.

Reading the family tree

Identical twins start from one fertilized egg, but each cell picks up small DNA changes as it divides, which leave a lineage record. The team sequenced 10 tumor samples, 12 normal tissue samples from the twins, 11 placental samples and 12 placental samples enriched for cells that build the placenta. Every tumor sample from both girls carried the genetic signature of twin A. The cancer started in her, likely near the end of the first trimester, and crossed to twin B in a single transfer late in the second trimester. Twin B's tumors appear to descend from one cell. Those dates depend on an assumed mutation rate and are estimates.

The analysis turned up two more surprises. Normal blood cells moved the other way: about 75% of the cells in a spleen sample from twin A came from twin B. And the earliest cell lineages contributed unevenly, one branch mostly to twin A, another to both girls and the placenta, so the embryo did not split evenly.

Twin to twin transfer of leukemia has been documented before through shared placental circulation. This is a solid tumor, traced cell by cell. Study author Sam Behjati told New Scientist that sharing a placenta inevitably means sharing blood vessels. The method, the authors note, can be reused to study how other childhood cancers begin.