The MammaTyper gene expression assay is a CE-marked in vitro molecular diagnostic test which measures the mRNA expression levels of the four genes ERBB2, ESR1, PGR and MKI67 in surgical breast cancer samples and pre-operative biopsies to assign a tumor to a molecular subtype (Luminal A-like, Luminal B-like (HER2 positive or negative), HER2 positive (non-luminal) and Triple negative (ductal)). Here, ESR1 is linked to breast cancer.