PGR and breast cancer: As it is crucial to maintain the features characterizing the heterogenicity of breast cancer subtypes, we next sought to assess the stability of the four classical markers estrogen receptor α (ESR1, ERα), progesterone receptor (PGR, PR), human epidermal growth factor receptor 2 (ERBB2, HER2) and, marker of proliferation Ki‐67 (MKI67, Ki67) in our FN‐silk models.