  The observation of $Z_c(3900)$ by the BESIII collaboration in the invariant
mass spectrum of $J/\psi\pi^\pm$ in $e^+e^-\to J/\psi\pi^+\pi^-$ at the center
of mass 4.260 GeV suggests the existence of a charged $\bar D D^*+D\bar D^*$
molecular state with $I(J^P)=1(1^+)$, which could be an isovector brother of
the famous X(3872) and an analogue of $Z_b(10610)$ claimed by the Belle
Collaboration. We demonstrate that this observation provides strong evidence
that the mysterious Y(4260) is a $\bar{D}D_1(2420) +D\bar{D}_1(2420)$ molecular
state. Especially, we show that the decay of this molecule naturally populates
low momentum $\bar DD^*$ pairs and leads unavoidably to a cusp at the $\bar
DD^*$ threshold. We discuss the signatures that distinguish such a $\bar DD^*$
cusp from the presence of a true resonance.
