RPGR and retinitis pigmentosa: The photoreceptor outer segment is a specialized primary cilium while the retinal pigment epithelium is a ciliated monolayer epithelium, and variants in a number of ciliary proteins, including RPGR, cause retinitis pigmentosa (Ghosh et al., 2009; Parmeggiani et al., 2011; Wheway et al., 2014).