a transgene was designed to contain (from 5' to 3') the entire mouse keratin 6 (Krt6) gene, a viral 2A oligopeptide (to mediate ribosomal skipping), cre recombinase, another 2A oligopeptide, the cerulean variant of green fluorescent protein (GFP), and a bovine polyadenylation sequence. This sequence was followed by a frt-flanked neomycin (neo) resistance cassette. This transgene was microinjected into fertilized C57BL/6J oocytes and founder male 4 was bred to establish line 4.