A LoxP-4XPolyA-LoxP-H2B-tdTomate-FRT-Neo-FRT cassette was targeted to the start codon of kit locus with disruption of endogenous ATG by homologous recombination in ES cells. Flp-mediated recombination removed the neo selection cassette, leaving a single FRT site behind. Resulting mouse line was crossed with Protamine-Cre mice to remove 4xPloyA stop cassette. (J:227907)