Sequence in the endogenous tx locus that encodes amino acid residues 23-366 has been replaced with a Recombinase-Mediated Cassette Exchange (RMCE) cassette that contains a Avic\GFP3xP3.cUa marker flanked by loxP sites and inverted attP sites. GFP is expressed mainly in the cap and ligament cells in this allele, indicating that expression is influenced by tx regulatory elements in addition to the 3xP3 promoter sequence.