  Partial cubes are the graphs which can be embedded into hypercubes. The {\em
cube polynomial} of a graph $G$ is a counting polynomial of induced hypercubes
of $G$, which is defined as $C(G,x):=\sum_{i\geqslant 0}\alpha_i(G)x^i$, where
$\alpha_i(G)$ is the number of induced $i$-cubes (hypercubes of dimension $i$)
of $G$. The {\em clique polynomial} of $G$ is defined as
$Cl(G,x):=\sum_{i\geqslant 0}a_i(G)x^i$, where $a_i(G)$ ($i\geqslant 1$) is the
number of $i$-cliques in $G$ and $a_0(G)=1$. Equivalently, $Cl(G, x)$ is
exactly the independence polynomial of the complement $\overline{G}$ of $G$.
The {\em crossing graph} $G^{\#}$ of a partial cube $G$ is the graph whose
vertices are corresponding to the $\Theta$-classes of $G$, and two
$\Theta$-classes are adjacent in $G^{\#}$ if and only if they cross in $G$. In
the present paper, we prove that for a partial cube $G$, $C(G,x)\leqslant
Cl(G^{\#}, x+1)$ and the equality holds if and only if $G$ is a median graph.
Since every graph can be represented as the crossing graph of a median graph
[SIAM J. Discrete Math., 15 (2002) 235--251], the above
necessary-and-sufficient result shows that the study on the cube polynomials of
median graphs can be transformed to the one on the clique polynomials of
general graphs (equivalently, on the independence polynomials of their
complements). In addition, we disprove the conjecture that the cube polynomials
of median graphs are unimodal.
