CD44 and neoplasm: Among the stem cell markers, CD24 were expressed in all of the CTC clusters at varying levels, while 5/6 clusters were found to express high levels (>50 rpm) of CD44. It has been previously reported that CD44 homophilic interactions and subsequent CD44–PAK2 interactions mediate tumor cell aggregation42 and improve stemness, survival, and metastatic progression43,44.