（Ruslan Dashinsky）

The sex of baby mice—and quite likely baby humans—is determined by a virus that inserted itself into the mammalian genome 1.5 million years ago, Live Science reports.

Yale researchers published their surprising findings on March 30 in Nature. According to a press release, more than 40 percent of the human genome is comprised of remnants from viruses that invaded it millions of years ago.

Researchers found some of that viral material on mice X chromosomes—the chromosomes that determine the sex of embryos. As long as that viral remnant is active, males and females are born at equal rates.

But if the viral remnant is turned off, which UPI reports happens in some embryos, males are born at twice the rate of females. While it's unclear if the same process holds for humans, the human X chromosome contains the same viral material as its mouse counterpart.

What's more, researchers believe the method by which mice embryos turn off the viral remnant could be used to keep cancer from spreading. That's because the same viral material is also found in tumors and could be helping them grow.

But really they're still trying to figure this whole thing out. "Why mammalian sex ratios are determined by a remnant of ancient virus is a fascinating question," says esearcher Andrew Xiao.

在人类进化的过程中，人类基因所拾获的DNA外在片段，大多来自古老的病毒。

据合众社报道，这些DNA外在片段大多是无效的，但有少数可以起到新作用，比如确定发育胚胎的性别。

美国耶鲁大学的科学家们认为古老病毒的DNA链在关闭X染色体时起到了关键作用，该DNA链在150万年前进入了哺乳动物的基因组中。

耶鲁大学干细胞中心的遗传学家萧琢（Andrew zhuo Xiao）是发表在《自然》上的这一最新研究成果的通讯作者，他表示为什么哺乳动物的性别比例会由一段残留的古老病毒来确定，这是令人感到着迷的问题。

耶鲁大学的科学家们经过研究确定有些胚胎关掉了与X染色体有关的病毒，从而影响了性别比例。这样的机制可以让关闭病毒，静默X染色体的表达，从而确保雄性的出生率达到50%。当分子标记正常时，雄性和雌性的出生数量会均等。

在这一最新研究发表之前，科学家们都认为哺乳动物可以通过操纵核苷酸胞嘧啶（nucleotide cytosine）来抑制基因表达。

萧琢和他的同事们认为最新发现的这一机制可以用来抑制癌症，因为癌症通常是通过劫持病毒来进行传播的。除了胚胎之外，人们还发现这一病毒在肿瘤和神经元中保持活跃的状态。