VSX1 and posterior polymorphous corneal dystrophy: In 2002 VSX1 mutations were first reported in PPCD and KC patients [234], in which two mutations (R166W and L159 M) were originally identified in KC patients. VSX1 encodes a pair-like homeodomain protein which binds to the core of the locus control region of the red and green visual pigment gene cluster and may regulate expression of the cone opsin genes during embryonic development [235, 236].