Notum (palmitoleoyl-protein carboxylesterase) complexed with inhibitor ARUK3004470 , molecular model. The image shows a ribbon rendering of notum with the inhibitor shown as spacefilling element-coloured molecule. Notum is a carboxylesterase that acts as a key negative regulator of the Wnt signalling pathway by specifically mediating depalmitoleoylation of WNT proteins.