A loxP site flanked neomycin resistance gene and STOP cassette was inserted into intron 1. Exon 8 was replaced with a modified exon 8 in which a C-to-T substitution results in the amino acid substitution of proline with serine at position 275 (p.P275S). This mutation corresponds to a proven UVB exposure mutational hostspot at human codon 278. Cre-mediated recombination removed the neo/STOP cassette.