Mixed-lineage leukemias are a group of hematologic malignancies that manifest as acute lymphoblastic, myeloid, or biphenotypic leukemias. They are caused by chromosomal translocations involving the MLL gene at the 11q23 locus, which lead to expression of chimeric proteins that fuse the N-terminus of the MLL protein to the C-terminus of one of more than fifty partners. MLL leukemia primarily develops in patients less than one year of age or in cases of therapy-related leukemia, and is associated with poor outcomes when treated with conventional cytotoxic drugs and bone marrow transplantation. Therefore, new molecular-based therapies are needed to target this group of leukemias The synthetic peptide PFWT was designed by our lab to disrupt the interaction between two common MLL fusion partners, AF4 and AF9. These proteins have been isolated as components of a transcription elongation complex whose other members include the RNA polymerase II regulator pTEFb and the chromatin modifying enzyme DOT1L. We have hypothesized that MLL fusions with AF4 or AF9 mistarget this complex to MLL target genes, whose expression contributes to the leukemia phenotype. We previously showed that PFWT demonstrates preferential toxicity toward MLL-rearranged leukemia cell lines. However, microarray studies did not identify any changes in gene expression of known MLL target genes in the sensitive cell line MV4-11 after PFWT treatment. Moreover, further experimentation has revealed that PFWT causes rapid cell membrane damage and actin cytoskeletal disruption in treated cells. We also demonstrate for the first time that the fusion partner AF9 is concentrated in the cytoplasm of the PFWT-sensitive cell line MV4-11. These findings have led us to propose a function for AF9 in actin cytoskeletal stability, which may also explain our observations with respect to the cytotoxic effects of the AF9-targeting peptide PFWT