However, Wnt/β-catenin signaling must also be increased in Dkk1+/− mice with reduced levels of the Wnt antagonist Dkk1 and in mice with a loss of function of Axin2, a negative regulator of the Wnt/β-catenin signaling, both of which also show a coloboma phenotype (Lieven and Rüther, 2011; Alldredge and Fuhrmann, 2016). Here, AXIN2 is linked to coloboma.