Lately I keep hearing the same sentence from women who had DCIS or are at an increased risk for breast cancer: “Well, I’m on Duavee.”

When I ask why, the answer usually traces back to one study and one number. The number is Ki-67. It went down, so the drug must be safe. Maybe even protective.

I understand why that feels reassuring. But a drop in Ki-67 is not the same as a cancer that shrinks, stays away, or never comes back. And if medicine is going to use Ki-67 to steer decisions for women, it has to use it the same way every time.

So let’s talk about what Ki-67 actually is, what it can tell us, and where medicine has stretched it further than the evidence goes.