  The implementation of the heavy quark spin symmetry in possible molecular
states at the mixed bottom-charm threshold is somewhat different from that at
the open charm or bottom thresholds. In particular it depends on two parameters
describing separately the symmetry for the bottom and for charmed quarks. The
corresponding spin structures of the $S$-wave molecular states of the meson
pairs $B^{(*)} D^{(*)}$ are discussed as well as their consequences for
properties of possible near-threshold resonances, analogs of the known
charmonium-like and bottomonium-like $X$ and $Z$ states.
