- TNBC is defined as staining by immunohistochemistry (IHC) < 1% or Allred score < 2 for estrogen receptor (ER) and progesterone receptor (PgR), and 0 or 1+ by IHC for human epidermal growth factor receptor 2 (HER2) or negative for gene amplification (average HER2 copy number < 4 signals/cell; HER2:CEP17 ratio < 2.0).