  We analyze experimental data of nuclear structure-function ratios
F_2^A/F_2^{A'} and Drell-Yan cross section ratios for obtaining optimum parton
distribution functions (PDFs) in nuclei. Then, uncertainties of the nuclear
PDFs are estimated by the Hessian method. Valence-quark distributions are
determined by the F_2 data at large x; however, the small-x part is not obvious
from the data. On the other hand, the antiquark distributions are determined
well at x~0.01 from the F_2 data and at x~0.1 by the Drell-Yan data; however,
the large-x behavior is not clear. Gluon distributions cannot be fixed by the
present data and they have large uncertainties in the whole x region.
Parametrization results are shown in comparison with the data. We provide a
useful code for calculating nuclear PDFs at given x and Q^2.
