Prompted by our previous finding that MAPK1p.E322K could demonstrate heightened sensitivity to erlotinib in vivo reminiscent of the patient’s clinical response in the exceptional responder, we generated isogenic tumor xenografts expressing MAPK1-WT, MAPK1p.D321N, and MAPK1p.R135K, and compared their in vivo erlotinib responses. This evidence concerns the gene MAPK1 and neoplasm.