CD44 and breast neoplasm: Consistently, an anti-CD81 activating agonist promoted breast tumor cell clustering in a CD81- and CD44-dependent manner as the cluster-enhancing effects of the antibody diminished in both CD81KO and CD44KO cells (Figure 4—figure supplement 4E), further demonstrating the interplay and cross-dependence between CD81 and CD44 for optimal cluster formation.