And the Ubiquitin is essentially linked to the enzyme via a thio ester bond. So on enzyme number one, on enzyme one, E one, we have a cysteine. And the side chain of that cysteine is used to attach that ubiquitous molecule. So let's take a look at the following diagram to see exactly what we mean. So, we have our Ubiquitin in its nonactive form. So in step one, we take an ATP molecule and we essentially transfer an amp from the ATP onto the Ubiquitin and we release a Pyrophosphate.