RB1 and neoplasm: PR-B expression determined by PLA strongly correlated with the levels of total PR by immunohistochemistry, consistent with previous reports.18,20 In the 44 cases negative for PR by immunohistochemistry, there was a correlation between PR-B expression determined by PLA and ER:PR-B interactions suggesting the presence of low levels of PR-B in the tumours was sufficient to interact with ER.