Cucurbitacin I (JSI-124) is a novel selective triterpenoid that acts as a potent inhibitor of the Janus kinase 2/signal transducer and activator of transcription 3 (JAK2/STAT3) signaling pathway with anti-proliferative and anti-tumor properties. Cucurbitacin I specifically suppresses levels of tyrosine phosphorylated STAT3 in v-Src-transformed NIH 3T3 cells and in A549 cells (IC50 = 500 nM) resulting in inhibition of STAT3 DNA binding and reduced STAT3-mediated gene transcription. It also suppresses JAK2 phosphorylation but does not affect Src, ERK, JNK or Akt. In nude mice, cucurbitacin I (1 mg/kg/day) suppressed the growth of various tumors expressing constitutively active STAT3.1 It promotes the differentiation of dendritic cells and macrophages and enhances the effect of cancer immunotherapy. Cucurbitacin I (1 µM for 2 hours) reduced clonogenicity of nasopharyngeal carcinoma cells in vitro and suppresses tumor growth in mice (1.3 mg/kg).