RPGR and cone dystrophy: Despite otherwise apparently unaffected RPGR structure, intact basic domain, and normal level of glutamylation which unsurprisingly preserve rod function, our study cannot exclude that the extended tail itself (via a dominant negative effect) negatively affects the folding and structure of RPGR protein preventing the binding to other proteins and eventually, interfering with the normal trafficking of cone opsins leading to the cone dystrophy, also observed with a similar RPGR extension (44).