The present invention relates to peptides which impair PBX-dependent regulation of gene transcription. In particular, the invention provides the use of a peptide comprising the amino acid sequence X.sub.1 X.sub.2 X.sub.3 W M X.sub.4 X.sub.5 X.sub.6 X.sub.7, wherein the sequence X.sub.1 to X.sub.7 is an amino acid sequence comprising at least 9 amino acids, which may optionally be interrupted by one or two amino acid residues between one or more of the 9 amino acid positions defined herein; X.sub.1 is selected from W, T, PE, KQI, VV, PQT, H, RI and absent; X.sub.2 is an amino acid with an aromatic side chain; X.sub.3 is P or D; X.sub.4 is an amino acid with a basic side chain; X.sub.5 is an amino acid with a charged side chain; X.sub.6 is an amino acid with a charged side chain; and X.sub.7 is an amino acid with a basic side chain or Serine; in the manufacture of a medicament for treating or preventing a disorder in which aberrant cell division occurs.