RB1 and breast cancer: We previously reported a requirement for PR-B expression and the presence of ER/PR/PELP1 and ER/PR/IGF1Rβ complexes at specific steroid receptor target genes, including CTSD26 in response to oestrogen stimulation; CTSD expression is associated with increased risk of relapse and metastasis.37 Given PR-B K388R cells lost the expression of IGF1Rβ, we asked whether either the IR or IRS-1 instead bind to the CTSD promoter in ER+ breast cancer cells expressing either wt or hyperactive K388R PR-B.