RB1 and malignant peripheral nerve sheath tumor: First, studies of cultured MPNST cells demonstrated RABL6A is an essential regulator of MPNST cell proliferation and survival in vitro.19 RABL6A was found to function, at least in part, by activating oncogenic CDK4/6 through inhibition of p27, thereby disabling RB1-mediated tumor suppression in tumor cells.