Notum (Palmitoleoyl-protein carboxylesteraseprotein), molecular model. The image shows a ribbon representation of notum, a carboxylesterase that acts as a key negative regulator of the Wnt signalling pathway by specifically mediating depalmitoleoylation of WNT proteins. The Wnt signalling pathways are a group of signal transduction pathways which begin with proteins that pass signals into a cell through cell surface receptors.