Lastly, in the Wnt/β-catenin loss of function mutant (K14-rtTA::teto-Cre::Ctnnb1fl/fl) (Nguyen et al., 2006; Perl et al., 2002), HF development is defective but Sox9 expression is expanded to the extent that all remaining cells in HFs express Sox9 (Figure 5H-J). This evidence concerns the gene SOX9 and hydrops fetalis.