Although I thought ER was a clear minority of Her2+, recent bits suggest a 50/50 split:
Lapatinib plus Letrozole as First-Line Therapy for HER-2+ Hormone Receptor–Positive Metastatic Breast Cancer
Quote:
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Approximately 50% of HER-2+ breast cancers are hormone receptor (HR)+, [4, 5, 7, 8] and HER-2 positivity is considered a marker of estrogen resistance [9, 10]. Therefore, dual treatment for tumors that overexpress both receptors is a logical approach.
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for more on the Her2/ER crosstalk and ability of Her2+/ER- to turn ER+ after Her 2 therapy:
HERE