A construct, containing the tdTomato red fluorescent marker gene and a neomycin resistance gene cassette, was inserted immediately downstream of the ATG translation start site in exon 1. The construct replaces the coding region of exon 1 and exon 2a. Stop codons were introduced in all three reading frames of exon 2b. RT-PCR and immunohistochemistry experiments confirmed the lack of transcript and protein expression in the retina from this allele.