MAPK3 and neoplasm: Immunohistochemical staining revealed that phosphorylated mitogen-activated protein kinase/extracellular signal-regulated kinase 1/2 (P-MAPK/ERK) was strongly expressed in both PanINs and tumor cells, suggesting activation of the MAPK/ERK growth pathway in the LSL-KrasG12D/+; LSL-Trp53R172H; Pdx-1-Cre pancreatic lesions but not in histologically normal mouse pancreas (Figure 3C).