A floxed Pgk-neo-bpA cassette was inserted into the Eco RI site into the preserved third intron in reverse orientation. An improved YFP, Citrine, coding sequence was inserted into the fourth exon as an in-frame fusion after the furin cleavage site that generates the mature peptide. The floxed Neo cassette was removed through crossing with Sox2-Cre transgenic animals. Although no endogenous sequence was deleted, this allele was determined to be functionally null allele.