Fig. 1. Nonhypomorphic, conditional reporter alleles of Fgf8 used for domain-specific ablation experiments. (A) Different cassettes were inserted into the 3' UTR of Fgf8 to generate the Fgf8^APN and Fgf8^GFPN alleles. An in-frame splice acceptor and the alkaline phosphatase (AP) or green fluorescent protein (GFP) reporter genes (green boxes) were positioned downstream of the 3' loxP site (red arrowheads) and an frt-flanked neo^r gene (green bars flanking labeled arrow) in a SpeI site located in the 3'UTR of Fgf8. These alleles are hypomorphic because the presence of neo^r. Flp-mediated recombination (purple arrow) of frt sites (green bars) deletes the neo^r gene and generates nonhypomorphic conditional reporter alleles, Fgf8^AP and Fgf8^GFP. Fgf8^AP and Fgf8^GFP alleles are inactivated with respect to production of functional Fgf8 message when Cre (large red arrow) recombines the loxP sites (red arrowheads) to delete exon 5 (Moon and Capecchi, 2000). Recombination results in expression of the AP or GFP reporter gene under control of Fgf8 regulatory sequences (Fgf8^APR, Fgf8^GFPR), allowing detection of functionally relevant recombination of Fgf8 (i.e. inactivation of Fgf8 in cells in which it is expressed).