ESR1 and cancer: The apparent positive prognostic influence of PR expression in ER-positive cancers may be due to robust ER activity in these cancers, implied by the fact that PR is an ER target gene, and in addition, by the fact that strong PR expression, as detected by immunohistochemistry, may denote, counter-intuitively, a less transcriptionally active protein, given that transcriptional activity is coupled with proteasomal degradation.