The interleukin 17 (IL-17) family proteins comprise six members (17A through 17F). However, recent studies found that co-expression of IL-17F and IL-17A in HEK293 cells results in the formation of biologically active IL-17F/IL-17A heterodimers, in addition to the IL-17F homodimers and IL-17A homodimers. Activated human CD4+ T cells were found to produce the IL-17A/F heterodimer, along with the corresponding homodimers. IL-17A was most potent, followed by IL-17A/F heterodimer, then IL-17F (100fold lower than IL-17A). Activated human CD4+ T cells in culture were found to secret IL-17F homodimer at 10-fold higher levels than IL-17A homodimer, suggesting that the majority of the IL-17A protein expressed exists in the form of the IL-17A/F heterodimer.