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. 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.