A protein binds to a single small molecule ligand. When the protein (50 pM) is mixed with the ligand (4 nM) and the solution is allowed to reach equilibrium, the proportion of protein molecules bound to the ligand is 10%.

From this single data point, estimate the equilibrium dissociation constant (K_d) for this interaction.

Type the answer as a number in units of nM but do not type the units or any other additional characters (e.g., type 10 rather than 10 nM).