Abstract
Colon cancer metastasis to the breast is a rare presentation and has a poor prognosis. Diagnosis of the primary site for the metastasis is usually aided by immunohistochemistry but can be non-conclusive. Gene expression assay can be helpful in diagnosing the primary cancer site. This is a 67-year-old female who presented with breast cancer metastasis of colorectal carcinoma origin. Immunohistochemistry was positive for cytokeratin AE1/AE3 cocktail and negative for ER, PR, CK20, CK7, CDX-2, and GATA-3, which was non-specific for any site of origin. The primary site was later identified using the gene expression assay CancerTYPE ID (BioTheranostics, Inc., San Diego, CA) which suggested likely primary was colon adenocarcinoma. This case showcases a rare presentation of colon cancer metastasis to the breasts and how gene expression assay can help us find the primary origin of cancer metastasis when we have an unclear diagnosis from immunohistochemistry.