Recent clinical trials revealed that prior use of Cetuximab may interfere with ICB efficacy. In this study, STAT1 protein levels changed after continued Cetuximab treatment (500 ug/ml). We used LC-MS/MS to compare the phosphorylation sites in wild-type and resistant cells. We proposed that the phosphorylation of STAT1 may lead to the degradation of the STAT1 protein.