Cadherin 23 and protocadherin 15 defects underlie two genetic forms of Usher type 1 (USH1) syndrome. (A, B) Three classes of cadherin 23 isoforms (a, b, and c in A), and four classes of protocadherin 15 isoforms (CD1, CD2, CD3, and SI in B) have been reported. The distinct PDZ domain-binding motifs in the different protocadherin 15 cytodomains (CD) are indicated. SI, secreted isoform (see Ahmed et al. 2008b). (C) Scanning electron micrographs of OHC hair bundles in wild-type and protocadherin 15-deficient mice at postnatal day 15 (P15). Note the fragmentation of the hair bundles in the mutant (Source: M. Leibovici, Institut Pasteur, France). (D) Summary of the interactions between USH1 proteins. Bar = 1 µm.