With the help of casein kinase 1 (CK1), which carries out an initial phosphorylation of β-catenin, GSK-3β is able to phosphorylate β-catenin a second time. This targets β-catenin for ubiquitination and degradation by cellular proteasomes. This prevents it from translocating into the nucleus, where it acts as a transcription factor for proliferation genes.[14] APC is also thought to be targeted to microtubules via the PDZ binding domain, stabilizing them.[15] The deactivation of the APC protein can take place after certain chain reactions in the cytoplasm are started, e.g. through the Wnt signals that destroy the conformation of the complex[citation needed].