PCDH15 and Usher syndrome: In the case of apical microvilli, the specific microvillar protocadherin PCDH15 was identified in human genetic studies of Usher syndrome, an inherited deaf-blindness disease caused by defects in stereocilia of the human ear cochlear cells and microvilli of the eye photoreceptor cells (Alagramam et al., 2001a,b; Ben-Yosef et al., 2003).