CDK4 and breast cancer: The results of another study revealed that in palbociclib-resistant ER+ breast cancer cells, the Cyclin D-CDK4/6 pathway could be reactivated, but it remained sensitive to mTOR inhibitor vistusertib, suggesting that the inhibition of the PI3K pathway through the inhibition of mTOR may be a potential option for patients who have relapsed on therapy with CDK4/6 inhibitors.