CCR7 and melanoma: CCL21a is a chemokine that promotes metastasis of many tumor types, including melanomas, breast, thyroid, colon, head, and neck cancers, via pairing with CCR7 expressed on tumor cells.[13, 29] Release of CCL21a from the gel diverted the TdLN migrating tumor cells to hydrogel and thus blocked the TdLN metastasis theoretically.