MERTK and choroideremia: Several monogenic forms of retinal degeneration are also caused by mutations in genes that are expressed in the RPE; for example, mutations in MERTK (Gal et al., 2000), which cause retinitis pigmentosa, CHM (Cremers et al., 1990), which cause choroideremia, and MYO7A which cause Usher syndrome type 1B (Weil et al., 1995).