SA activity is abrogated in NF1 deficient cells by expression of the NF1-GRD domain. A, KR158 cells stably transduced with the NF1-GRD domain are resistant to SA, whereas KR158 cells transduced with the empty vector (PMSCV) are not, as measured by an XTT assay. Points, mean percentage growth of three wells compared to DMSO treated controls. B, NF1-GRD stably transduced KR158 cells do not show loss of stress actin fibers after 18 hours treatment with SA, in contrast to KR158 cells transduced with PMSCV empty vector. Cells were probed with Alexa Fluor 488-phallodin to visualize the actin cytoskeleton, and counterstained with DAPI to visualize the nucleus.