1 Introduction 1. 1 Wnts and Development The f amily of Wnt genes
consists of at least 19 members in vertebrates. Wnts ha v e multiple
roles during normal development and aetiology of diseases (Wodarz and
Nusse 1998; Moon et al. 2002, 2004). These genes encode for
glycoproteins that are released in the intercellular space, acting as
intercellular mediators (Moon et al. 1997). By binding to their special
receptors, the Frizzled (Fz) receptors, they are able to activate at
least three different pathways: the canonical, the non-canonical and the
Ca 2+ pathway (Fig. 1 ). The Wnt family has been subdivided, using
functional assays, into at least two subclasses. In Xenopus, Wnt1, Wnt3a
and Wnt8 are able to induce axis duplication in embryos, whereas Wnt4,
Wnt5a and Wnt11 cannot (Du et al. 1995). Those Wnts that are able to
induce axis duplication are thought to act canonically; those who cannot
are thought to act non-canonically. In the literature, therefore, some
Wnts are called canonical Wnts while others are called n- cano nical
Wnts, suggesting that a special Wnt always activates the same signalling
pathway. It is unlikely that the specificity dictating cellular
responses resides solely in the Wnt ligands because no one has reported
sequence or structural motifs in Wnts that predict their activities in
these cellular and embryonic assays.