ERBB2 and breast cancer: In a subgroup analysis based on the molecular BC subtype, the detection of cluster mCTCs or cluster PD-L1+ mCTCs was associated with reduced OS among hormone-receptor-positive patients only (n = 18; p = 0.004 for both CTC subsets), but not in the triple-negative patient cohort (n = 11; p = 0.268 and p = 0.634, respectively), whereas HER2-positive patients (n = 5) had not detectable any clusters of these specific phenotypes.