  The non-conservation of charmed-strange current in the neutrino deep
inelastic scattering ($\nu$DIS) strongly affects the longitudinal structure
function, $F_L$, at small values of Bjorken $x$. The corresponding correction
to $F_L$ is a higher twist effect enhanced at small-$x$ by the rapidly growing
gluon density factor. As a result, the component of $F_L$ induced by the
charmed-strange current prevails over the light-quark component and dominates
$F_L=F_L^{cs}+F_L^{ud}$ at $x\lsim 0.01$ and $Q^2\sim m_c^2$. The color dipole
analysis clarifies the physics behind the phenomenon and provides a
quantitative estimate of the effect.
