A peptide antagonist of the transcription factor CCAAT/enhancer-binding protein beta (C/EBP beta), with potential antineoplastic activity. Upon administration, C/EBP beta antagonist ST101 targets and inhibits the activity of C/EBP beta. This prevents the expression of C/EBP beta target genes and proteins, including the anti-apoptotic protein B-cell lymphoma 2 (Bcl-2), cyclins and inhibitor of differentiation (ID) family of proteins, which are involved in cell proliferation, differentiation, and cell cycle regulation. This may lead to apoptosis in tumor cells. C/EBP beta is overexpressed in many cancers and plays an important role in the regulation of cell differentiation; its expression is associated with tumor cell survival and proliferation.