The expression pattern and biological function of IL-17E are the opposite of those of IL-17A, GM-CSF, and TNF and are not specific to Th17 cells; thus, IL-17A, IL-17F, IL-21, IL-22, and IL-26 were defined as signature cytokines for representing the infiltration level of Th17 cells in breast cancer tissue. Here, IL21 is linked to breast cancer.