  The scale dependence of the ratios of parton distributions in a proton of a
nucleus $A$ and in the free proton, $R_i^A(x,Q^2)=f_{i/A}(x,Q^2)/f_i(x,Q^2)$,
is studied within the framework of the lowest order leading-twist DGLAP
evolution. By evolving the initial nuclear distributions obtained with the
GRV-LO and CTEQ4L sets at a scale $Q_0^2$, we show that the ratios
$R_i^A(x,Q^2)$ are only moderately sensitive to the choice of a specific modern
set of free parton distributions. We propose that to a good first
approximation, this parton distribution set-dependence of the nuclear ratios
$R_i^A(x,Q^2)$ can be neglected in practical applications. With this result, we
offer a numerical parametrization of $R_i^A(x,Q^2)$ for all parton flavours $i$
in any $A>2$, and at any $10^{-6}\le x \le 1$ and any $Q^2\ge 2.25$ GeV$^2$ for
computing cross sections of hard processes in nuclear collisions.
