Fig. 1.

a Proposed mammary epithelial differentiation hierarchy based on fractionation and transplantation studies, comprising quiescent¹⁰ and activated stem cells,⁸ the latter of which are marked by PROCR. b PROCR⁺ CSCs, present in ~50% of TNBCs, can regenerate an entire tumor that recapitulates the parental tumor while PROCR⁻ cells lack tumor-forming potential upon transplantation. Treatment of mice harboring PROCR⁺ tumors with a specific anti-PROCR nanobody leads to profound tumor regression