OBJECTIVE Over-expression of transforming growth factor β1 (TGF-β1) has been observed in many advanced cancers.The present study was aimed at developing potential antisense oligonucleotides (ASONs) to repress TGF-β1 expression in rhabdomyosarcoma (RMS) RD cells, and to examine their effect on tumorigenicity of RD cells in vivo.METHODS ASONs targeting the region surrounding the start codon of TGF-β1 were synthesized and transferred into cells in the form of complexes with Lipofectamine 2000. The TGF-β1 protein was determined by immunofluorescence and ELISA.The cell viability and cell cycle were examined by MTT and flow cytometry. The RD cells, with or without TGF-β1ASON, in 50 μl of serum-free EMDM medium were injected subcutaneously into the right flank of nude mice. The tumors were then measured and weighed.RESULTS The ASON sequence targeting the first start site at bases 841-855 of the human TGF-β1 gene had the greatest effect on attenuating the expression of TGF-β1 (P