It remains unclear how the signals of mutant KRASG12 in the transformed cells spread to the surrounding non-mutated cells and changes the microenvironment to promote tumor formation. We identified that Williams-Beuren syndrome transcription factor (WSTF), a non-secretory protein, was released in complex with secretory proteinneuregulin- 3 (NRG3). The KRASG12 mutant activates the transcription of NRG3. The WSTF/NRG3 in extracellular space could activate oncogenic pathways in normal colon cells carrying wild type KRAS and endow them with the ability to express NRG3 and release WSTF/NRG3. Extracellular WSTF/NRG3 promotes the formation of colon tumors. Blockade of extracellular WSTF could restore cetuximab sensitivity of colon cancer cells with mutant KRAS. The appearance of WSTF/NRG3 in serum and urine correlates with a colon tumor carrying a KRASG12 mutant. In summary, our demonstration provides a new pathway to our understanding of the biological development of complex diseases.