CCL17 and neoplasm: Furthermore, Treg cells are involved in tumor development and progression by suppressing tumor immunity; Treg cells can be activated by chemokines (e.g., CCR4-CCL17/22, CCR8-CCL1, CCR10-CCL28, and CXCR3-CCL9/10/11), are chemotactically attracted to the tumor microenvironment, and participate in microenvironment regulation (106–108).