In situ labelling of ER with [125I]TAZ. Breast tissues slices (samples 1-4, ER positive; sample 5, ER-negative) were incubated with 1 nmol/l [125I]TAZ for 1 h at 0°C and the unbound ligand was removed. Then, all tissues were mixed with Krebs-Ringer phosphate buffer containing 1% SDS, 1.6 mmol/l EDTA and 2% β-mercaptoethanol, and briefly homogenized. After lysis at 100°C, proteins were extracted with phenol, precipitated by acetone and were finally analyzed by SDS-PAGE. The figure shows the electrophoretic patterns of these tissue ERs.