def plot_number_of_structures_per_kinase_pdb_pair(structures):
    """
    Plot the number of structures that have x structures per kinase-PDB ID pair.

    Parameters
    ----------
    structures : pandas.DataFrame
        Structures DataFrame from opencadd.databases.klifs module.

    Returns
    -------
    matplotlib.pyplot.axis
        Plot axis.
    """
    kinase_pdb_pair_sizes = structures.groupby(["kinase.klifs_name", "structure.pdb_id"]).size()
    ax = kinase_pdb_pair_sizes.plot(
        kind="hist",
        title="Number of structures per kinase-PDB pair",
        bins=kinase_pdb_pair_sizes.max(),
    )
    ax.set_xlabel("Number of structures per kinase-PDB pair")
    ax.set_ylabel("Number of kinase-PDB pairs")
    return ax