Male DNA is commonly found in the brains of women, a study has found.
The cause of the phenomenon is most likely being pregnant with a boy, say scientists.
No one yet knows the medical implications of the discovery. But there is a suggestion that male DNA in the female brain might protect against Alzheimer's disease.
Other kinds of "microchimerism", the harbouring of genetic material and cells swapped between foetus and mother during pregnancy, have been linked to both beneficial and harmful effects.
A study of 59 deceased women aged 32 to 101 found that the brains of those with Alzheimer's were less likely to contain foetal-derived male DNA. The genetic material was also seen in lower concentrations in regions of the brain most affected by the disease.
But the scientists stressed that the small number of women studied and largely unknown pregnancy history means that no firm conclusions can be drawn from these findings.
Study leader Dr William Chan, from the Fred Hutchinson Cancer Research Centre in Seattle, US, said: "Currently, the biological significance of harbouring male DNA and male cells in the human brain requires further investigation."
The researchers detected male microchimerism in 63% of the brain specimens. Male DNA was distributed in multiple brain regions and appeared to persist throughout life. The oldest woman whose brain contained male foetal DNA was 94.
The findings, published in the online journal Public Library of Science ONE, suggest that foetal cells frequently cross from the bloodstream to the brain.
Previous studies have shown that in some conditions, such as breast cancer, cells of male foetal origin may be protective. In others, such as colon cancer, they have been associated with increased risk.
Research has also found a lower risk of rheumatoid arthritis, an autoimmune disease, in women who have given birth to a son at least once.