Routine immunohistochemical examination of the right breast cancer. (a) Majority of tumor cells represent a strong immunoreactivity for estrogen receptor (ER). ER immunostain. (b) Most tumor cells show a positive reaction for progesterone receptor (PGR). PGR immunostain. (c) Only a weak and heterogeneous positivity is observed in <10% of tumor cells, which are known to be negative at HercepTest score 0. HER2/neu-immunostain. (d) Approximately 30% of tumor cells are positive for Ki-67. Ki-67 immunostain. The scale bars indicate 100 μm.