RPGR and Duchenne muscular dystrophy: When XK gene segment deletions are excessive, they may involve contiguous XK genes, such as the CYBB and RPGR genes, which could result in “Xp21 DNA microdeletion syndrome”, including XLRP, chronic granulomatous disease, Duchenne muscular dystrophy, and ornithine transcarbamylase deficiency (OTC) (Wilcox et al., 1986; Watkins et al., 2011; Roulis et al., 2018).