PDCD1 and breast cancer: In immunocompetent female C57BL/6 mice, M/D-driven tumors recapitulate multiple immunobiological features of HR+HER2− breast cancer in women, including a scarce immune infiltrate [43], pronounced resistance to immune checkpoint inhibition with programmed cell death 1 (PDCD1, best known as PD-1) blockers [43], and exquisite sensitivity to CDK4/6 inhibitors [24], representing a unique model for translational studies.