MAPK3 and cancer: However, in contrast to the EGFR-MEK-ERK1/2 pathway, the MKK4-JNK-cJun pathway can be defective in human cancers due to loss-of-function MKK4 mutations [42,43,44,45,46], and JNK negatively affects breast tumorigenesis and mammary cell motility and migration [47].