Before revision:  Approximating the probability density function (PDF) of \Delta \psi_N by a log-normal distribution, the PDF of \Delta \psi _{N (\simeq \Delta\Delta \psi_{ND}) in fixed mutants is estimated.
After revision:  Approximating the probability density function (PDF) of \Delta \psi_N by a log-normal distribution, PDFs of \Delta \psi _{N (\simeq \Delta\Delta \psi_{ND}) in fixed mutants is estimated.