Five MAP2K1 gene mutations (encodes the MEK1 kinase) were detected in either drug-resistance specimens (3 mutations; MEK1V60E, MEK1G128V, and MEK1V154I in Patients 41, 32, and 28, respectively)… including MEK1V60E and MEK1G128V. These results raised the possibility that somatic MAP2K1 mutations might promote resistance to RAF inhibition in some contexts but not others.