  The non-factorizable terms in the operator product expansion have been
recognized as one of theoretical obstacles for precision determination of the
mixing parameter $V_{ub}$ from semileptonic B decays. It is pointed out here
that the recent CLEO data on the parameters of the heavy quark expansion
$\lambda_1$ and ${\bar \Lambda}$, combined with a theoretical bound on
$\lambda_1$ strongly favor the existence of a sizeable contribution of
non-factorizable terms in semileptonic decays of $D$ mesons. Thus these terms
are likely to solve the long-standing problem of the deficit of semileptonic
decay rate of the $D$ mesons, and with better data their magnitude can be
determined and used in studies of the parameter $V_{ub}$.
