A fragment that encompasses the BEAF-32 transcription unit, with upstream sequence extending to -900bp relative to the ATG codon of the BEAF-32 A isoform. The C-terminal end of the BEAF-32 coding sequence (sequence common to both isoforms) is tagged with mRFP1 and with Tag:NLS(SV40-largeT). Sequence extending from approximately -30 to +140 relative to the ATG codon of the A isoform has been deleted. Sequence corresponding to amino acid residues 157-198 of the B isoform has been deleted. This results in a transgene which expresses only the mutant B isoform (tagged with mRFP1).