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Two identical sisters came into the world carrying the same rare cancer. They lost their lives to an undifferentiated sarcoma, a soft-tissue cancer driven by a unique gene fusion (MN1::ZNF341). The story began before they were born, when pregnancy imaging picked up a tumor on Twin A's face. Then came the birth, when cancer deposits were found in both Twin A and Twin B. Researchers now had to work out how the cancer reached both girls. Did the cancer start on its own in each sister, or did it begin in one and pass to the other while they were still in the womb?

The researchers sequenced the entire genetic code of healthy and tumor tissues, along with placental samples, to trace the cancer's journey between the twins. All tumor samples from both twins carried a distinctive genetic signature belonging to Twin A.

The cancer likely began as a facial tumor in Twin A at the end of the first trimester, which was visible on scans before birth. The cancer then crossed the placenta to reach Twin B late in the second trimester.

Beyond tracing the cancer's spread, the findings challenged the classical model of how identical twins develop from an early embryo. These findings are published in Nature Communications.

Identical twins share the same starting point, developing from a single fertilized egg, but it is extremely unusual for both to develop the same cancer.

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