Three mice in a cage, and each has her own schedule. One sniffs the bedding, another gnaws a grain, the third freezes by the water bottle. A month later, one of them will get an injection that temporarily stops her internal clock, while the other two continue living by the schedule. This is how an experiment begins in which a customs checkpoint for genes tests how a single drug changes ovarian aging.

Tamoxifen works like a customs officer who holds passengers at the border for five days and then lets them go. In mouse ovaries, this drug switches on the desired genes, but it also pretends to be a hormone. The ovary is so sensitive to hormonal signals that any hiccup at customs can cause a jam lasting several months. The scientists gave mice tamoxifen at three months of age, then tracked their cycles until one year.

At six and twelve months, the ovaries were compared under the microscope and by RNA composition. Age left its usual marks: connective tissue grew, and large cells with multiple nuclei accumulated. But in mice that had gone through customs, these marks were the same as in those that had lived without delays. Only three genes in the young mice and three in the old ones changed their activity after tamoxifen, and none of them is linked to aging.

Now biologists have a clear rule: after tamoxifen, you need to wait three months before drawing conclusions about the ovaries. It's like knowing exactly when customs will clear all the queues, so you can walk through to the exit. The work fills an important gap, because previously no one had checked whether the drug itself leaves long-term traces in the organ. Today this knowledge helps plan reproductive aging experiments, and tomorrow, perhaps, ovarian cancer studies as well, since that disease often starts in the neighboring tubes.

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