de Kruijff IE, et al. - Given the potential clinical significance of the androgen receptor (AR) in metastatic breast cancer (mBC) as it might be a treatment target and has been associated with endocrine resistance, researchers assessed AR mRNA expression in circulating tumor cells (CTC-AR) and in matched primary tumor samples from mBC patients representing different breast cancer subtypes. Furthermore, CTC-AR-status was determined in relation to the outcome on endocrine therapy. In CellSearch-enriched CTCs from 124 mBC patients and in 52 matched FFPE primary tissues, they measured AR, and 92 AR or estrogen receptor (ER) related genes, using quantitative reverse-transcriptase PCR. Outcomes suggest the possibility of determining AR in RNA isolated from CTCs, within the set 31% AR-positive samples. As AR status in CTC samples was discordant to corresponding primary tumors, they suggest a possible utility of determination of AR expression in CTCs as a promising tool to select mBC patients for AR inhibiting agents.