def hill_coefficient_exp(tf_name,approx=False):
"""What is the effective hill coefficient of a binding site?"""
motif = getattr(Escherichia_coli,tf_name)
pssm = PSSM(motif)
real_copies = copy_dict[tf_name][0]
approx_copies = copy_dict[tf_name][1]
copies = real_copies if not approx else approx_copies
ns = []
x_ks = []
for site in motif:
site_energy = pssm.trap(site)
xs = copies
ys = map(lambda mu:fermi_dirac(site_energy,mu),mus)
plt.plot(xs,ys)
x_k,n = fit_hill_function(xs,ys)
print site,site.operon,n,x_k
x_ks.append(x_k)
ns.append(n)
#plt.semilogx()
#plt.show()
return ns,x_ks
