Estrogen-positive breast cancers are often treated with anti-estrogen therapies. But about half of these cancers contain a subpopulation of cells marked by the protein cytokeratin 5 (CK5), which resists treatment — and breast cancers with higher CK5 expression have poorer prognosis. These cells, which have characteristics of stem cells, often survive treatment to drive or even restart cancer growth. Previous work has shown that retinoic acid, a chemical that results from the body’s natural breakdown of vitamin A, should act against these CK5+ cells, but clinical trials of retinoids against breast cancer have been largely unsuccessful.