A CD-cassette (291bp sequence consisting of the 3' end of intron 2 of the α1 tubulin gene of C.reinhardtii (nucleotides 46 to 116) which contains splice branch and acceptor sites, an open reading frame (nucleotides 118 to 172, the 'guest exon') and sequences derived from intron 2 of the α2 tubulin gene of C.reinhardtii (nucleotides 173 to 241) which contain a splice donor site) have been inserted into Ubx intron 3. Expression is driven by UAS regulatory sequences and the open reading frame includes the Tag:HA tag. The result is two chimeric introns surrounding the new 'guest exon'. When Ubx is transcribed, spliced and translated, a protein containing a 'guest peptide' insert is produced.