MAPK1 and plasma cell myeloma: Here, we test the applicability and efficiency of the system in its constitutively active and doxycyclin-inducible forms by collectively knocking down the four components of the MEK/MAPK module (the proteins MAP2K1 (MEK1), MAP2K2 (MEK2), MAPK3 (ERK1), and MAPK1 (ERK2)) in transient and stable transfections of multiple myeloma cell lines.