Forkhead box I3 (FOXI3) is a transcription factor expressed in pre-placodal ectoderm adjacent to the anterior neural plate, and later in the ectoderm and endoderm lining the pharyngeal arches. FOXI3 is required for the development of the entire inner ear, and for branchial arch neural crest survival, patterning, and the subsequent development of branchial arch derivatives. Foxi3CreER knock-in/knock-out (KI/KO) mice have bicistronic expression of a CreER fusion protein and enhanced green fluorescent protein (eGFP) driven by the Foxi3 endogenous promoter, while abolishing endogenous Foxi3 expression. See below for more information on low levels of eGFP expression. While heterozygous Foxi3CreER KI/KO mice are viable and fertile, homozygous Foxi3CreER KI/KO mice die at or before birth. All homozygous Foxi3CreER KI/KO mice demonstrate severe craniofacial defects and a small percentage (~20-25%) also exhibit exencephaly. Restricted to the cytoplasm, Cre-ERT can only gain access to the nuclear compartment after exposure to tamoxifen. When Foxi3CreER KI/KO mice are bred with mice containing loxP-flanked sequences, tamoxifen-inducible Cre-mediated recombination will result in deletion of the floxed sequences in the cre-expressing cells of the offspring. Cre-recombinase activity was verified by the donating laboratory by crossing these KIKO mice to an Rosa26-Ai9, mouse line (Stock No. <a href="https://www.jax.org/strain/007909">007909</a>). After tamoxifen administration, resulting progeny demonstrated tdTomato expression in the ectoderm and endoderm of the first three cleft and pouch regions as well as in multiple progenitor cells in the pre-placodal region, at embryonic age 10.5 (E10.5). At embryonic age 14.5 (E14.5), tdTomato expression was also noted in the ectoderm of the external ear pinna and ear canal, in the endodermal derivatives of the third and fourth pharyngeal arches and in hair and tooth placodes. The donating laboratory has indicated that although the Foxi3CreER KI/KO allele has functional CreER protein expression, the low level of expression from the construct did not allow detection of the eGFP protein, either by native fluorescence or by immunostaining with GFP antibodies. This strain may be useful as a tool for tracking embryonic craniofacial development as well as tooth and hair development.

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Foxi3CreER knock-in/knock-out mice have a CreER fusion protein driven by the Foxi3 endogenous promoter, while abolishing endogenous Foxi3 expression. This strain may be useful for Cre-lox studies of Foxi3-expressing cells during embryogenesis; specifically, neuroepithelial cells, pharyngeal arches and pouches, as well as hair and tooth progenitor cells. Although eGFP is also present in this Foxi3CreER allele, its low level of expression from the construct did not allow detection of the eGFP protein, either by native fluorescence or by immunostaining with GFP antibodies. The donating laboratory has also made a Foxi3 reporter line available that retains endogenous Foxi3 expression: Foxi3GFP (Stock No. <a href="https://www.jax.org/strain/038586">038586</a>).

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