CYP1B1 and coloboma: While large disruptions in gene function resulting from frameshift or nonsense mutations lead to these widespread ocular defects,52,53 rare missense PAX6 mutations have been associated with colobomas.54 In mice, Pax6 regulates other genes associated with colobomas, such as Vsx2 (Chx10), Maf1, and Six3. 55, –57 The current studies showed that in zebrafish, cyp1b1 overexpression increased shha gene expression in the midline floor plate, while decreasing pax6a, pax6b, and vsx2 gene expression in the developing retina.