In China, a woman was found dead from a gunshot. Investigators found an unexpected clue during the autopsy. The victim's body showed the Y chromosome, which exists only in males. At first, they suspected that cells from a male suspect had remained in the victim's body. But that was not it. The Y chromosome was not a trace from outside; it was inside the victim's own cells.
According to a paper published early this year in an international forensic science journal, the victim had lived as a complete woman until her death. Her sex had not been ambiguous in life, and she had even given birth to a son. But her hair carried XY, the male sex chromosome set. In the kidneys, female XX cells and male XY cells appeared in similar proportions. Two different sets of genes coexisted within one person's body. Scientists call such a person a "human chimera."
The chimera is originally a monster from Greek mythology. It has the head of a lion, the body of a goat, and the tail of a serpent. In 1830, a massive Roman-era mosaic was discovered in Autun in the Burgundy region of eastern France. It depicted Bellerophon on the winged horse Pegasus killing the chimera with a spear. The name of the mythical monster has become a scientific term today, referring to a living organism that carries two or more genetic lineages in one body.
Traces of a twin found in court
Chromosomes are structures in which DNA, the genetic material, is wound around proteins called histones and condensed. The name chromosomes came from the fact that, when observing structures inside the cell nucleus under a microscope, they take up dye well. Human cells have 23 pairs of chromosomes. Twenty-two pairs are autosomes found in both males and females, and the remaining pair are sex chromosomes. Females have XX, and males have XY sex chromosomes. In the Chinese woman's body, XX and XY were both identified. She was born female and lived as a woman, but part of her body was genetically male.
Scientists judged that she likely had been a chimera from the earliest stages after fertilization. If two sperm enter one egg, a triploid embryo with three sets of chromosomes can form. Such embryos usually cannot develop normally, but during early development the chromosomes can re-segregate, allowing some cells to survive with XX and others with XY. In this case, the fact that the XX and XY cells shared the same X chromosome was interpreted to mean that both cell lines originated from the same egg.
Human chimeras became known to the world through a courtroom. It was the case of Lydia Fairchild in the United States. In 2002, Fairchild, a 26-year-old living in Washington state, applied for child support for two children she had with her estranged husband and for a third child soon to be born. To verify identities, welfare officials swabbed the inside of the mouths of the mother and children to collect DNA. The tests showed the children's father was indeed the husband, but the mother was not Fairchild.
Fairchild was swiftly branded a child-support fraudster. She submitted photos showing her pregnant with the two children and as they grew, but these were useless against the DNA results. The court even sent staff to the delivery room for the third child. The staff confirmed that Fairchild herself gave birth and collected blood immediately after delivery. Yet DNA testing again showed the third child was not Fairchild's biological child.
The twist began when her lawyer found a medical paper that year describing a similar case. A woman named Karen Keegan also underwent DNA testing to receive a kidney transplant from her children, only to learn genetically she was not their mother. Researchers explained that Keegan had absorbed cells from a fraternal twin who died in utero, and that the twin's genes remained and were passed on to her children.
It was proposed that Fairchild might be the same type of case as Keegan. DNA testing was conducted again. This time, cells were collected not only from the inside of her cheek but also from her blood, skin, hair, and cervix. Tests showed DNA from blood and skin did not match the children, but another DNA profile that connected to the children was identified in the cervix. In Fairchild's body remained the genes of a fraternal twin sister who had disappeared in the fetal stage.
In the end, Fairchild was the biological mother who had given birth to the children, but by DNA she was their "aunt." That was because part of her ovaries carried the twin sister's genes, which were passed on to the children. Fairchild's own DNA did not go to the children. The court acknowledged its error and dismissed the case. DNA is powerful evidence, but human chimeras showed that such evidence is not always straightforward.
Children's cells left in a mother's brain
Human chimeras first appeared in a medical paper in 1953 in the case of "Mrs. Mc." During a blood donation, it was found that she had two blood types, A and O. One blood type was hers, and the other was a trace of her twin brother, who died at 3 months old. This means it was not a case like Fairchild's, who absorbed twin DNA in the uterus. Scientists explained that during fetal life, blood cells passed between the twins in the mother's womb, leaving both blood types.
Broadly speaking, any human may be a chimera to some degree. During pregnancy, fetal cells move into the mother's body, and the mother's cells are also transferred to the fetus. This is called microchimerism. In 2012, researchers at the University of Washington School of Medicine reported that in autopsies of the brains of 59 women, they found cells with the male Y chromosome in 37 of them. The team interpreted these cells as originating from sons the women had given birth to. It suggests children's cells can remain in a mother's body for life. If it has always felt like a mother can read one's mind, perhaps it is because part of oneself exists inside her body.
Chimerism is likely to be detected more often in the future. As in vitro fertilization becomes more common, multiple embryos are often transferred to improve the chances of pregnancy. When multiple embryos develop in the same uterus, in theory there is a greater likelihood that cells will mix or that one embryo will absorb another.
Of course, most chimeras live without knowing they are chimeras, because there is no outward sign. But there are exceptions. Taylor Muhl, an American singer-songwriter and model, said on a TV talk show in 2017 that she is a human chimera. She revealed her body to raise awareness about chimerism. The skin on the left and right sides of her abdomen differed in color. One side looked more reddish. Muhl explained that because of the DNA of a twin absorbed in fetal life, her immune system shows an autoimmune response, attacking part of her own body as if it were foreign tissue.
Chimera embryos bring hope for organ transplants
Chimeras have revealed the complexity of human genetics and a blind spot in DNA forensics. They could also become a key to solving organ shortages. Research is underway on chimera embryos that combine human and animal cells to create transplantable organs. In June last year, Chinese researchers said at the International Society for Stem Cell Research that a pig chimera embryo with human heart cells survived for 21 days.
Pigs have organ sizes and shapes similar to humans and have been noted for decades as an alternative to the shortage of transplantable organs. But no matter how similar, because they are pig organs, immune rejection can occur once inside the human body. Growing human organs in a pig's body could avoid this problem.
The researchers reprogrammed human skin cells into induced pluripotent stem cells (iPS cells). iPS cells are primitive cells that can grow into any human cell, but unlike embryonic stem cells, they can be made from a patient's own cells without destroying eggs or embryos. The team deleted two genes that grow the heart in pig embryos and injected human iPS cells. If the chimera embryo grows in the uterus of a surrogate pig, the other organs will be pig-derived, but the heart will have human characteristics.
Animal experiments have already confirmed the possibility of organ transplantation using chimera embryos. In 2017, a team led by Nakauchi Hiromitsu at Stanford University in the United States grew a mouse pancreas inside a rat, then transplanted it into diabetic mice. The transplanted mice were cured of diabetes. A path opened to create needed organs using the bodies of other animals. In myth, the chimera became a sacrificial offering for the birth of a hero. Now the day is approaching when it itself transforms into a hero that treats disease.
References
Forensic Science International: Genetics(2026), DOI: https://doi.org/10.1016/j.fsigen.2025.103394
Nature(2025), DOI: https://doi.org/10.1038/d41586-025-01854-x
Cell(2021), DOI: https://doi/org/10.1016/j.cell.2021.03.020
Nature(2017), DOI: https://doi.org/10.1038/nature21070
PLOS One(2012), DOI: https://doi.org/10.1371/journal.pone.0045592
New England Journal of Medicine(2002), DOI: https://doi.org/10.1056/NEJMoa013452
British Medical Journal(1953), DOI: https://doi.org/10.1136/bmj.2.4827.81