Constitutive activation of MEK1 or MEK2 promotes tumor metastasis to the liver and lung. (A) Motility of IEC-6 cell populations was assayed by measuring their migration through porous membranes coated with collagen type I in modified Boyden chambers. Cells on the bottom side of the membrane were fixed and stained. (B) Invasiveness of IEC-6 cell populations was assayed as in A except that membranes were coated with a layer of Matrigel extracellular matrix proteins. Results are expressed as fold-increase relative to vector-infected cells. (C) Incidence of metastasis following orthotopic transplantation of IEC-6 cell populations in the caecum of mice. (D) Representative pictures of lung metastases derived from intestinal tumors expressing activated MEK1 or MEK2.