ESR1 and breast carcinoma: Furthermore, MAPK alterations were substantially enriched in patients with polyclonal ESR1 mutations as compared to single ESR1 mutations (39.3% versus 19.6% respectively, p = 0.0004, Fig. 2b), identifying a subset of oestrogen receptor (ER) positive breast cancers that develop polyclonal genomic resistance (Fig. 2c).