ESR1 and breast carcinoma: To elucidate the mechanisms involved, we focused on the PKA-dependent phosphorylation of BIG3 as a key event occurring after E2 stimulation in breast cancer cells, as in silico analysis revealed that BIG3 contains two potential RII-binding domains (RIIBDs) that bind to the RII subunit of PKA (318–338 and 1,225–1,245 amino acids; Supplementary Fig. 1g), similar to BIG1 and BIG2 (refs 10, 11), and PKA is highly expressed in all ERα-positive breast cancer cell lines (Supplementary Fig. 1h).