TTBK1 (tau tubulin kinase 1) is a serine/threonine kinase that is localized to the cytoplasm and expressed at high levels in brain and at lower levels in testis and spinal cord. It can phosphorylate Tau (a protein involved in tubulin polymerization) on threonine, tyrosine and serine residues. Phosphorylation of Tau causes its aggregation and subsequent loss of function, suggesting an important role for TTBK1 in the control of tubulin dynamics.