  The neutron structure function F2n(x) is evaluated within the kinematic range
10**(-3)<x<1 from the deuteron and proton data by employing relativistic
theoretical description of F2D(x) and several assumptions on the hihg-x
asymptotics of F2n(x)/F2p(x). It is shown that new measurements of F2D(x) in
the range 0.6<x<0.8 would substantially improve understanding of the relation
between d and u valence quarks in the limit x -> 1.
