In classic signaling, binding of IL-6 to membrane IL-6R recruits gp130 to form an IL-6/IL-6R/gp130 hexamer complex. In trans-signaling, proteolysis and/or alternative splicing generates soluble form of IL-6R (sIL-6R), which binds to IL-6 and can stimulate gp130-expressing cells. Both classic and trans-signaling pathways activate downstream signaling including JAK-STAT, SHP-2-Ras-Raf-MEK-ERK1/2, and PI3K-AKT, leading to the transcription of target genes important for cell survival, proliferation, differentiation, immunomodulation, inflammation, angiogenesis, metastasis, and tumorigenesis.