This could potentially be explained by the fact that, although E- and P-selectin have both been previously linked to the lung metastasis of different cancer types [40,57,58,59], evidence supports a particular role for the interaction of soluble E-selectin with breast cancer cells to promote metastatic behavior [56,58,60], particularly in the context of CD44+ breast cancer cells [58,60]. This evidence concerns the gene CD44 and breast cancer.