Estrogen receptor and proteolytic activity in human breast tumor nuclei

We have previously found that human breast cancer cells in tissue culture contain estrogen receptor in their nuclei despite the absence of estrogen. We have now investigated solid human breast cancer biopsies and find that proteolytic activities in extracts from the nuclear pellets of these biopsies interfere with or prevent the measurement of nuclear estrogen receptor when the protamine assay is used. However, the problems of receptor degradation can be avoided by the use of a hydroxylapatite assay.