Essential for Spemann-Mangold organizer formation and subsequent anterior head development in the embryo. Inhibits canonical Wnt signaling pathway by antagonizing nuclear import of beta-catenin (ctnnb1) during embryogenesis. Interacts (via NLS1 and NLS2) with dvl2; the interaction is negatively regulated by Wnt stimulation. Interacts with csnk1a1. Interacts with ctnnb1; the interaction is positively regulated by Wnt stimulation. Expressed maternally and then widely throughout embryogenesis. During the neurula stage, highly expressed in the neural plate and neural fold. Strong expression also seen in brain, eyes and spinal cord at the tadpole stage. Phosphorylated by ck1/csnk1a1. Morpholino knockdown results in embryos with small eyes, head and brain structures. Named Custos for the Latin term for 'guard' or 'keeper'. Belongs to the CUSTOS family.