In 44 tumours that were negative for PR by immunohistochemistry (Allred score 0–2; representing weak positive staining in less than 1% of tumour cells), the median number of ER:PR-B interactions was 2.28 (inter-quartile range 1.8–10.3), significantly lower than the number of interactions in cases with a PR Allred score of 3 or more (median 6.45 signals per cell, inter-quartile range 0.26–7.99, p = 0.001) (Fig. 2b). This evidence concerns the gene RB1 and neoplasm.