CD44 and neoplasm: Using tissue sections from MBC patients and triple-negative BC patient-derived xenograft (TNBC PDX) models, this study revealed the ubiquitous expression of CD44 (a marker for cancer stem cells [28,29,30,31] and a prognostic predictor in multiple cancer [32,33,34,35,36,37]) in CTCcls compared to single CTCs, and reported the interaction between CD44 and its downstream target p-21 activated kinase 2 (PAK2) as a key mechanism found to mediate tumor cell aggregation [27].