STAT3 and thyroid gland undifferentiated (anaplastic) carcinoma: When anaplastic thyroid cancer cells were treated with leptin in the presence or absence of specific signal transduction pathway inhibitors PD98059, LY294002 and an inhibitor of STAT3, S31-201, the leptin-induced cell invasion was inhibited by PD and S31-201, but not LY294002 (Figure 4B).