Shown below is a binding curve for a drug binding to a single binding site on a target protein.

Using information from this curve, estimate the concentration of the drug required such that 90% of the protein molecules will be bound to the drug.

Your answer should be given in units of nanomolar (nM).

Type only numbers (do not type the units or any other additional characters)

Hint: if you're struggling here because the graph doesn't go high enough, think about what information you can glean from the graph and take it from there.