ESR1 and cancer: In the DESTINY-Breast04 trial, 90% of the participants were ER+.1 Multiple studies also showed that about half of all ER+ cancers are HER-low with IHC.3-8 For early-stage ER + breast cancers, the Oncotype DX Recurrence Score (RS) assay is used to guide adjuvant chemotherapy.9 The Oncotype DX also provides standardized HER2 mRNA levels by RT-PCR.