Together, these findings suggest the presence of a distinct poor-prognosis phenotype within HR+/HER2− breast cancer, which can be distinguished by three overlapping characteristics, high proliferation index (Ki-67 > 25%), poor tumor differentiation (grade 3), and low PR expression (<20% or <4 Allred score), which is hereafter referred to as the “Luminal B poor-prognosis” (LPP) phenotype. This evidence concerns the gene PGR and breast cancer.