  The spectrum of heavy-hadron molecules is constrained by heavy-quark symmetry
in its different manifestations. Heavy-quark spin symmetry for instance
connects the properties of the ground and excited states of heavy hadrons,
while heavy-antiquark-diquark symmetry connects the properties of heavy
antimesons ($\bar{D}$, $\bar{D}^*$) and doubly heavy baryons ($\Xi_{cc}$,
$\Xi_{cc}^*$). A prediction of these symmetries is that if the $P_c(4450)$ is
indeed a $\bar{D}^* \Sigma_c$ bound state, then there should be a series of
$\bar{D}^* \Sigma_c^*$, $\Xi_{cc} \Sigma_c$, $\Xi_{cc}^* \Sigma_c$, $\Xi_{cc}
\Sigma_c^*$ and $\Xi_{cc}^* \Sigma_c^*$ partners. The concrete application of
heavy-quark spin symmetry indicates that, if the $P_c(4450)$ is a
$\frac{3}{2}^{-}$ $\bar{D}^* \Sigma_c$ molecule, the existence of a
$\frac{5}{2}^{-}$ $\bar{D}^* \Sigma_c^*$ partner with similar binding energy
--- which we call $P_c(4515)$, given its expected mass --- is likely.
Conversely, the application of heavy-antiquark-diquark symmetry indicates that
the $0^+$ $\Xi_{cc} \Sigma_c$, $1^+$ $\Xi_{cc} \Sigma_c^*$, $2^+$ $\Xi_{cc}^*
\Sigma_c$ and $3^+$ $\Xi_{cc}^* \Sigma_c^*$ molecules are likely to bind too,
with binding energies in the $20-30\,{\rm MeV}$ range.
